The message you're replying to specifically said not vegan. Your points about farming might be good but the vegan stuff is a straw man.

The best diet for health and the environment[0] is high in whole grains, vegetables, fruit, nuts, seeds, with small amounts of meat and sugar. Making meat a treat (as written above) is absolutely right.

[0] https://www.theguardian.com/environment/2019/jan/16/new-plan...