1. Humans routinely make the mistake of thinking that what works for them will work for everyone.
2. The world is full of assholes who give people a really hard time for being the weirdo doing the weird thing, even if it isn't hurting anyone at all, so people want everyone to do it to try to defend against the negative consequences of being the weirdo that people will give hell to for simply being weird.
And yet even vegetarians can't all get on the same page, which is part of why we even have the world vegan to distinguish some subset of extremists from the rest of them. Similarly, there are myriad sects of Christianity because even people who all agree that they believe in Jesus Christ can't get on the same page about a lot of things and Islam is an extreme example of that: Just as Christianity grew out of Judaism yet it seen as distinct from being Jewish, Islam grew out of Christianity.
A man who grew up Muslim told me Jesus Christ is one of the major saints of Islam. It does not entirely eschew the ideas of the Christian Bible, it just believes in and celebrates a more recent prophet, the Prophet Muhammad.
Muhammad ...was an Arab religious, social, and political leader and the founder of Islam. According to Islamic doctrine, he was a prophet, sent to preach and confirm the monotheistic teachings of Adam, Abraham, Moses, Jesus, and other prophets.
https://en.wikipedia.org/wiki/Muhammad
I have zero fear that all of humanity or even the majority of humanity will start consuming an entirely beef-based diet any time soon. I think it is high time we got over our delusions that all humans everywhere will ever be on the same page at the same time on any given topic and embrace the fact that people eat different, people express spiritual beliefs differently and so forth and it's actually okay and for the best to let some people have their extremist diet without treating them terribly for finding something that works for them and without acting like that needs to become the new norm for everyone else as well.
So if someone comes and recommends an all-meat diet as being healthier for almost everyone than the current plant+meat based diet; and someone else points out that it would be an environmental disaster; and you talk about how local farming can mean that meat eating is not necessarily that bad for the environment - then your argument is only relevant if it is still true when a significant proportion of the population follows this new diet. Hence my extrapolation to ~7B people following said recommendation.
To call a diet sustainable or unsustainable, it only makes sense to talk about its impact IF it caught on for a significant proportion of the population for whom it is recommended. There is no unsustainable diet if we're only thinking of a few people following it. I could recommend an all-Saffron diet and you could call it sustainable if only 10 people have to follow it.
So again, an all-meat diet, even if it were actually healthy , would be an unsustainable recommendation to make for the general population. It may well be a very good recommendation for people with specific conditions, and it could be made sustainable if those people were a tiny minority of the population. For the general population, the only possible diet that doesn't bleed the earth dry is one that is mostly vegetarian, with some amount of meat maybe once a week or even more rarely.