There's a third aspect to that. Or maybe it's another ecology problem.
While I don't have the numbers, I strongly suspect there's not enough grass to feed all the humans the amount of meat they would like to eat right now. At least based on observing the prices of intensive vs grass fed, or watching the numbers for intensive production alone.
So if we want grass fed as the new normal, we're either going to have to intensify it back again, or we'll end up eating respectively less meat. That's near-vegetarianism or bust.
Yes this is true - there is not enough grasslands to feed the current demand for meat - actually there is not enough feedlots to feed the demand for meat on a global scale. Many poor people around the world would eat more meat if it was cheaper.
The solutions to let the market solve the problem - if there was only grass fed meat available the cost would be very high and meat would become a luxury again.
Assuming this is an honest question and not a lame attempt at a gotcha: as more people switch to a vegan diet we will gradually stop breeding them in the first place, so there are no animals for something to "happen to."
It's also not the point of ecological vegetarianism: it's preserving the diversity of natural ecosystems, as opposed to food farms.