Sure; and I actually don't think that it's very hard to slightly revise the argument that you posted for the non-human animal case so that it captures precisely this - you just have to be careful about which humans and which non-human animals are relevant where. For instance, I think that this is a straightforward way to do it:
P1. If Entity A deserves some particular degree of moral consideration and Entity B does not deserve that same degree of moral consideration, there must be some relevant difference between the two entities that grounds this difference in moral status.
P2. For each sentient non-human animal, there are sentient humans from whom they do not differ in any such relevant respects.
Therefore, C. Each sentient non-human animals deserves a degree of moral consideration similar to that of some sentient human.
I think that's actually a very nice, simple, 2-premise version of Singer's Argument from Less Able Humans.
Sure, I agree that in some contexts it may well be good to foreground arguments from economic efficiency & sustainability, but that doesn't mean that arguments about non-human animal well-being and its moral importance are completely unimportant for all contexts. Arguments about non-human animal well-being can:Margaret Hayek wrote: ↑Sat Dec 09, 2017 12:35 pm (with animals I think the best arguments we currently have come from economic efficiency and sustainability, not moral logic). I like Schwitzgebel's argument (found elsewhere in his work) that we ought to take other beings "at their word" (in other words, if an animal exhibits signs of pain, it's best to assume they actually feel pain until proven otherwise. Similarly, if animals appear to be "subjects of a life" it's best to take that at face value until we have more information that might disprove this.
(1) add heft to the overall case for veganism - including additional heft to the moral importance of environmental sustainability, since environmental damage harms wild animal victims very, very pervasively,
(2) help explain why veganism should be prioritized among the possible changes one should make in one's life. It is true that veganism is probably the most important thing most people can do to prevent environmental damage, and it carries the lowest opportunity costs (indeed, between the financial benefits - only systematic study I know of showed healthy vegan diets to be $750 per year cheaper than healthy non-vegan diets) and the health benefits, the opportunity costs of being vegan are probably in most cases negative - i.e. it's a strict complement to any other form of effective altruism). But again, (i) consideration of the moral importance of non-human animals (including wild ones) boosts the importance of avoiding environmental damage, and (ii) the direct harms to farmed animals may still be important in explaining why veganism is even more important than comparable environmental damage mitigating activities. In this context what Tobias Leenaert calls "full Monty" arguments that actually challenge speciesism (like that above - at least if we interpret "similar to" in the conclusion as meaning "equally sized interests count equally"), which aren't necessary to convince people that they should be vegan and can risk unnecessarily alienating potential vegans / vegetarians / reducitarians, may actually be good to prevent recidivism among vegans. I think that Andy Warski's explanation to Isaac and Richard of his experience in contrast to others may be very instructive here. Andy explained how he came to go vegan for the animals, and stopped being vegan because he didn't care enough about the animals (and had rationalizations about why non-human animals weren't that important) to ride out the difficulties he encountered in adjusting to a vegan diet. This is in contrast to the experience of others, who see equally sized harms to animals as in themselves as important to avoid inflicting as equally sized harms to humans (e.g. suffering to any of us, or death to intellectually disabled humans), and for whom a return to carnism is almost unthinkable, no matter how hard it is to adjust to veganism,
(3) help explain why some forms of harming non-human animals to obtain animal products that have smaller environmental damage footprint are still important to avoid. Most importantly, let's be honest: among land agriculture, the worst offender against animal well-being is chicken / poultry farming (e.g. the suffering is most intense, and as many of us know out of the ~28 land animals consumed annually per year, it's about 26 chickens, 1 turkey, .1 cow and .5 pig), and the worst offender against the environmental is probably cattle agriculture (massive water use, land use, feed conversion at something like 16:1 instead of chickens and pigs which are more like 3:1). Yes, poultry agriculture is still of course inefficient and environmentally destructive relative to veganism, but it's NO WHERE NEAR as bad as cattle agriculture; and the bulk of the figures for why going vegans reduces your climate emissions so much have to do with cattle agriculture (including both beef and dairy). I encountered this in a big way, with my sustainability office advising groups to cut out beef rather than to cut out animal products: in many cases this will just increase poultry consumption, which is far worse in animal well being terms. I can of course make arguments about why poultry is still bad, but I don't have as much to go on as if I can appeal to animal well-being. There are also likely some very low impact ways of obtaining animal products, like certain forms of hunting in areas populated by many wild animals, very small-scale fishing in areas populated by many fish, and perhaps some kinds of very rare, small scale animal agriculture (e.g. how bad are the climate emissions from fully grass fed goats? What about fully grass fed rabbits?).
(4) show how vegans who do care about non-human animal well-being - and let's be honest, it's probably the vast majority (I think in Nick Cooney's veganomics book he found that most actually go vegan/vegetarian either for the animals or for health - I don't think there were many who did it just for environmental reasons, although to be fair it was published in 2013, prior to the influence of Cowspiracy and such) - aren't just hysterically overcome by unseasoned emotion and perhaps appealing to health and the environment to rationalize their irrational over-emotional concern for animals. In this context, while very low impact ways of obtaining animal products (small scale hunting in some places, small scale fishing in others, etc.) may not be that important in and of themselves, vegans are likely to be asked about them, and if they don't have a good answer to why they think what they do about them, their message may make them seem less credible. I could be wrong, but it seems to me that the ideal solution is to have a good answer about why consistent concern for animal well-being explains why vegans aren't OK with very-low-environmental-impact ways of obtaining animal products, combined with a caveat "even if you disagree with us about this, we can both agree that it would be an improvement you should still stop consuming most of the animal products available to you, and consumed only these ones." I think that's going to be more effective than just saying "oh, yeah, that's right; when I argue for being vegan I just mean stop consuming the products of large scale animal agriculture (or maybe really just cattle agriculture); small scale deer hunting and line fishing is totally cool - or, at most, I have emotional or quasi-religious objections to it that I concede can't be defended by rational arguments."
I wonder if a good way to strike a balance between the risk of alienation but force of "full Monty" arguments (about why non-human animals' interests are just as important as comparable interests of comparable humans) and weakened arguments (about why if we just have some minimal consistent concern for non-human animals, veganism should follow), is to present the full Monty argument, then say that even if it's wrong, it seems so clearly right that the weakened version has to be right, and then combine that with empirics to argue for veganism. This is essentially the strategy outlined on the presentation of the Argument from Less Able Humans on the phil vegan wiki (http://philosophicalvegan.com/wiki/index.php/Less_Able_Humans).
In the context of the tweak of the argument you presented, I think this might look something like this. you first present a version of the argument above (a full-Monty argument that establishes moral parity between non-human animals and comparable humans), but maybe one with an additional premise to make the conclusion positively that non-human animals' interests are morally considerable:
P1. If Entity A deserves some particular degree of moral consideration and Entity B does not deserve that same degree of moral consideration, there must be some relevant difference between the two entities that grounds this difference in moral status.
P2. For each sentient non-human animal, there are sentient humans from whom they do not differ in any such relevant respects.
P3. These sentient humans (e.g. orphaned profoundly intellectually disabled humans about whom no one happens to care) DO deserve a degree of moral consideration: indeed, it's just as morally important in itself for us to avoid inflicting an equally sized harm (like intense suffering) on them as it is to avoid inflicting it on typical humans like us
Therefore, C. Each sentient non-human animals DOES deserve a degree of moral consideration: indeed, it's just as morally important in itself for us to avoid inflicting an equally sized harm (like intense suffering) on them as it is to avoid inflicting it on typical humans like us
To defend P2 you presumably have to consider how the only difference between profoundly intellectually disabled humans and non-human animals is bare biological species, and once you appreciate what that is (ability to interbreed and produce fertile offspring, psychology-independent morphology, phenotype-independent genotype, history of phylogenetic descent), it seems obviously morally irrelevant (as clearly irrelevant and for the same reasons as bare biological sex / bigger vs. smaller gamete and ethnicity / superficial face / skin / hair morphology related to areas of ancestry). You then say "look, even if there's an error in here somewhere, the differences between the relevant sentient humans and the relevant sentient non-human animals seems so irrelevant that there's no way they can be THAT relevant - because they seem clearly IRRELEVANT they can at most make a minimal difference. So even if P2 is wrong, we can at least be confident in:
(P2*) For each sentient non-human animal, there are sentient humans from whom they do not differ in any such relevant respects so radically, so that, if the humans' equally sized interests count in themselves just as much as ours, then the non-human animals' interests are AT LEAST so important that it's morally very important to avoid inflicting enormous harm on the non-human animals for at most relatively trivial benefits to us.
You then combine P1 and P2* with P3 to arrive at:
Therefore, C* Each sentient non-human animal's interests are AT LEAST so important that it's morally very important to avoid inflicting enormous harm on the non-human animals for at most relatively trivial benefits to us.
You then combine C* with the empirical premise:
P4. Consuming animal products (i.e. not being vegan) inflicts enormous harm on sentient non-human animals for at most relatively trivial benefits to us
to arrive at
C2. It's morally very important to not consume animal products, i.e. to be vegan.
All of the empirical stuff you mentioned about environmental degradation and economic efficiency, as well as health and the effects of farming conditions (and indeed small scale hunting and fishing) on non-human animal well-being go into defending P4.