The carnist claims that eating meat is somehow to be grandfathered in as though it is now ethical, simply because it was either:
The idea that it was universally necessary is a baseless assertion to start (e.g. Jainism has existed for hundreds of years which teaches non-violence towards animals). So then the fallback position would have to be that violence towards animals "was generally regarded as OK", which then is just really a popularity argument. This is a poor metric for morality, as history itself has shown. So if you believe that history should dictate morality, then you probably shouldn't make this argument in the first place.
The only way to lump in modern farming (throwing diseased animals in small cages filled with their own filth for one's own taste entertainment) with something like traditional hunter-gatherers (fighting to survive in the wild) is to cherry-pick the characteristics of each of those actions to an unrecognizable limit. This, therefore, isn't the justification that one can use to justify carnism in a modern context.
This is based on some kind of idealized parallel universe that can't possibly include McDonald's Drive Thru - which has not existed for thousands of years.
People have been doing a lot of things for a long time, including slavery, murder, theft, and torture. That doesn't just give one freedom from ethical responsibility in any other moral scenario.
The issue is that it's unethical to abuse animals for your entertainment under any ethical system. You can't counter that with "yeah but it's been going on for thousands of years". OK, great, well it's still unethical to abuse animals for your entertainment, and you've stated an irrelevant fact.