Just a reminder: the underlying issue is really 'how sustainable is the current global human population under business-as-usual practices'?
It's clear a plant-based diet will reduce the land area, water, and fertilizer needed for food production - but if human population grows in response, then you simply end up back where you started - but with the plant production going to feed humans instead of livestock.
There's also the problem of food distribution - many countries rely heavily on imports to meet basic food needs, and the producing countries are typically running high-impact agriculture - double-cropping high-efficiency strains soy, corn, wheat, potato, etc. crops using vast amounts of fertilizers and water (essentially the 'Green Revolution' approach). Abandoning high-intensity agriculture (going back to pre-industrial methods like crop rotation) would cut food production in half at least, leading to global famines. Hence, pushing towards regional independence on basic food production is pretty important, given the uncertain future of climate impacts on agricultural production.
A final thing to keep in mind: methane from animals and wetlands is quite different from methane from fossil fuel production, as the latter represents a net transfer of carbon from stable geological deposits to the atmosphere, while the former represents the cycling of atmospheric carbon (CO2) into plant matter, and while those carbon atoms may be burped out by cattle as methane (CH4), they end up oxidized back to CO2 within 10-20 years in the atmosphere. Regardless, cutting the global cattle population by 80-90% makes a lot of sense (this is about what pre-industrial popuations were like). Meat would then be quite expensive, a luxury item, which also makes sense.