I think historically, man did eat meat, but much less so.

And I guess it wouldn't have much ecological impact if everyone lived on their own little Earthship-like farm, with few cattle for a portion of meat once or twice a month.

It's true however that in today's civilization meat is incredibly resource-intensive and inefficient. However, then on the market it would simply mean it would be drastically more expensive.

Why is it not so? Is meat really subsidized to such a high degree? Maybe. And that is the cause of the problem then. I.e. we don't need no global vegan eco police if we just eliminated the root causes.