Climate crisis

In the short term that would be a lot more damaging to the environment because then we would have high volumes of livestock solely for the purpose of producing animal byproducts such as cheese, milk, eggs. That livestock needs to be renewed but, even with our best husbandry and genetic modelling, that means at least a third of all new livestock won't be productive by reason of being male. Of course we could just slaughter them but that seems a) utterly stupid and wasteful and b) leaves us with a very environmentally unfriendly carcass surplus.

There is also the massive problem that means that you'd be abandoning massive amounts of people to die a slow horrible death of malnutrition because they live in parts of the world where plant based agriculture simply isn't sufficient to sustain them without some kind of meat component. There alternatives to that such as expenditure of massive amounts of energy shipping in food or a collossal amount of forced repatriation. Both of which come with their own environmental and ethical questions relating to cultural abuse of those people you're imposing this change on.

Even then, the sudden decline in meat based diet across the whole global population probably isn't enough to mitigate the human impact on climate change. There are other easier, far more palatable (excuse the pun) things that we can do as a planet, countries, communities and individuals that will have far greater effects.