Is that so? I wonder what happens with a person if he/she grew with normal diet and was put on vegan? I am not sure this is entirely healthy...
The Dutch are the tallest nation on earth because they eat very protein rich stuff [diary products]. That was on the news not so long ago...and you know what - funny but taller people are more successful in life. It's probably due to cognitive bias, the same one that makes people forgive the trespasses of beautiful people more readily that those of ugly people. So there is something in us that values height and perhaps it is connected via something like [illustration only example] "tall person - has access to protein rich food - good mate".
Also funny that all vegetarians I know [not even vegans] are on average not very tall or wide and honestly will have a difficulty to compete with meat eaters if we were in the "kill or be killed" situation.
So, as the food supplies begin to dwindle [they will eventually as will everything in a finite system with infinite growing demand] we will see more and more propaganda to switch to "eco food" like insects, microorganisms ans so on. It totally amazes me when people nonchalantly say "well, if we want to feed the ever growing population we will have to switch the menu" and no no one says "why are we so stupid to continue our infinite growth".
So 3 billion meat eaters on earth or 6 billion vegetarians or 10 billion vegans or 20 billion fungi eaters [numbers purely for illustration] - why one is "better" than the other? The impact on the eco system would be the same. Actually more because those 20 billion might still want heating, clothing and mobile phones...
I think however, that one IS better than the rest - namely the largest number possible that gives everyone the richest and healthiest diet possible in a sustainable manner, according to who we are.
Who are we? We are omnivorous and without meat we won't be even here. I will not go into details, since my knowledge is not professional but I have heard many times that meat eating [also very important - fish eating] gave us the spare energy to grow the brains that made us human. Cooking the food was the next revolution. With more concentrated source of food you need less of digestive system so you can shrink it while expanding other parts of the body. I see in my mouth the cutters of the rabbit, the canines of the tiger and the molars of the cow. And I ain't going to argue with 4 billion years of evolution and will oppose those who not only fight it but do it for the wrong reasons.
In conclusion I would rephrase the OP. "Forcing or manipulating people or society to not be omnivorous [as we want, please, are entitled to and, if you are religious, were made to be] is dangerous, useless and often stupid and could be therefore be called unethical".