No Ethical Consumption Under Capitalism
The argument that there is no such thing as ethical consumption under capitalism, and that as such buying a veggie burger is equivalent in wrongness to buying a beef burger is a common argument among anti-capitalist meat eaters against going vegan.
This argument draws from one of two major logical fallacies, and several dubious empirical claims:
False Equivalence
A false equivalence is often drawn from mistakes such as denying the role of Individual Responsibility in a system where multiple parties are at play in harm. In this case the error is very similar, but rather than deem all parties innocent for lack of responsibility, all parties are deemed guilty for being part of the system and unjustly denied credit for attempting to do less based on the erroneous claim that it's completely ineffective. In either case, the denial is used as an excuse for people to behave in any way they prefer (such as consuming animal products) because either nobody is to blame or we're all guilty anyway no matter what.
Beyond this if the anti-capitalist recognizes that indeed individuals can have an effect and are owed credit for it, the claim may be made that indeed veggie burgers and beef burgers are precisely equal in harm. From the flawed perspective of Deontology, that claim can be at least somewhat consistent: in a deontological system wrong is wrong, and there are no inherent degrees or weights to different wrongs -- lying is as murder, and you can't lie to save somebody's life or even a billion people's lives. However, from any form of consequentialism (from utilitarianism to forms of rule consequentialism and virtue ethics) the claim of their equivalency is impossible to substantiate without severely distorting reality.
In the most objective environmental terms, there is no comparison. Burgers like the Beyond burger have been confirmed by independent study[1] to by substantially better on all meaningful metrics.
"Beyond Meat commissioned the Center for Sustainable Systems at University of Michigan to conduct a “cradle-to-distribution” life cycle assessment of the Beyond Burger, a plant-based patty designed to look, cook and taste like fresh ground beef. The purpose of the study is to compare environmental impacts – chosen here as greenhouse gas emissions, cumulative energy demand (energy use), water use, and land use – with those from typical beef production in the U.S. A secondary purpose is to highlight opportunities for improvement in the environmental performance of the Beyond Burger product chain and provide Beyond Meat with a benchmark against which improvement efforts can be measured. The primary audiences are both internal stakeholders at Beyond Meat as well as external customers, consumers, and interested stakeholders.[...]
Based on a comparative assessment of the current Beyond Burger production system with the 2017 beef LCA by Thoma et al, the Beyond Burger generates 90% less greenhouse gas emissions, requires 46% less energy, has >99% less impact on water scarcity and 93% less impact on land use than a ¼ pound of U.S. beef."
Considering other metrics makes the comparison even more dramatic. The metric anti-capitalists are often most concerned with, treatment of workers, overwhelmingly disfavors slaughterhouses and meat processing as some of the worst paid, stressful, and dehumanizing jobs in existence. Studies have evaluated workers in beef processing to find significantly higher rates of Serious psychological distress compared to the general population[2], and reports of PTSD, severe workplace injury, and even workers being denied bathroom breaks and forced to wear diapers are also common.
While anti-capitalists don't necessarily care about ecology, effects of animal agriculture on the conservation are also severe. Beyond the global environmental effects, clearance of land for grazing cattle and growing fodder is destroying the rainforests; it's reported that soy is a major responsible crop, but only a small fraction of that is fed to humans (most is animal feed). Animal agriculture has been identified as a leading cause of species extinction and the single largest cause of habitat loss[3]
Study after study have identifies vegetarian and vegan diets as fundamentally better for the world in every reasonable respect. And that is not even to get into Animal Cruelty.
In order for these two products to be equal or even close to equal, there would have to be some outstanding variable where vegan products cause so much harm as to outweigh everything else, and nothing is apparent or has ever been plausibly suggested, only anti-vegan fear mongering on health that goes against consensus of nutritional experts and pseudoscience of Regenerative Agriculture that claims cows improve the environment (something that has been consistently discredited).
Appeal To Futility
Just because it's impossible to avoid all wrong (futility) doesn't mean we have no duty to lessen harm. Any difference in harm between two options creates a moral imperative to choose the less harmful one.
Capitalistic Consumption Is Unethical
The claim that all consumption under capitalism is unethical (at least to some degree) to begin with begs the question: Is it? Unlike the case of Beef vs. Mock meats, the issue of capitalism is more complicated -- there are harms, yes, but also benefits to weigh against those harms. Most importantly, unlike the alternative to meat found in vegan food products (which have been proven to be sustainable) there is no obvious alternative to capitalism, and the theoretical ones are both fraught with issues and unproven.
It's very understandable for people in the Middle ages to consume animal products; there was no way for them to plausibly believe they could be healthy without them, nutritional science didn't exist, B-12 hadn't been discovered (much less any vitamins), there were no mock meats and no known examples of vegans living healthfully to model. Even though it WAS probably possible for people to be vegan if they ate and drank just the right things, it wasn't possible for them to know what those were so it wasn't actually possible for them given the knowledge they had.
There may be an ideal cooperative communistic model of government and economics we have yet to discover and prove, but for lack of that knowledge NOBODY carries the ethical burden of implementing it, and nobody is morally culpable for just participating in the current system -- not as long as we act consciously to chose those things we DO know are better options (like the veggie burger over the beef). Ethics are a product of choice, and choice is a product of knowledge. Until we learn more our burden is to continue learning, and that means implementing gradual progressive reforms and seeing what happens. Learning from the past, attempts at revolutions do not inspire confidence in radical and overnight change: from Communist Soviet Russia to Maoist China these attempts have all gone horribly wrong. You can always pull a no-true-Scottsman and complain it wasn't the right ideology or it was done for the wrong reasons, but the fact is that we don't know what is and perhaps any destabilization like that is inherently prone to abuse. Again, we don't know, and in this case ignorance is dis-empowering but also absolves us of the moral culpability for not abstaining from capitalistic consumption, which is thus not in any sense inherently unethical as long as we make the right choices we DO know about (IOW just choose the veggie burger, not the beef burger).