One thing I’ve never seen addressed is the question of fertiliser. Plants don’t just need water, they need fertiliser. Usually composted animal manure. If we don’t keep animals, where will that come from? This is not to say we should eat as much meat as is considered normal, just that mixed farming is a thing for a reason.
Also, I wonder if vegans consider their carbon footprint. I imagine you can do both low and high carbon footprint for both meat and vegan diets, so neither is the answer in isolation. Vegan stuff is often heavily processed, imported protein, grown in a pretty unsustainable way, and not automatically better than local grass raised beef. Vegan shoes, for example, are generally plastic.
Vegan monoculture is just as bad as any other monoculture. And pests still have to be kept off vegan food. I don't suppose anyone cares about various insects, rats or pigeons, but fluffy bunnies, cute deer and so on in the country will be being killed to keep you vegan diet, well, yours. And in other countries it’s likely to be more exotic animals. Elephants still need to be kept off your sugarcane, don’t they? Veganism isn’t automatically cruelty free.
As always, with any straightforward solution, it isn’t that simple. I think the move towards eating less meat is a really good thing, but Veganism on its own is not going to save the planet. I personally don’t believe it’s even possible for the world to become vegan, much less actually desirable.