I can understand Jesus and Peter advocating the eating of anything edible. 1st century Jerusalem probably did not have many vegetarian options in the first place.
But many of today's Christians and Muslims consider it mandatory to eat meat....which is difficult to understand.
I don't think a large part of cheese making involves rennet from butchered calves anymore...
It's not about rennet - or at least that's a tiny part of it. It's about the simple fact that cows have to be in calf to produce milk, and as with many mammals, the calves will be about 50% female and 50% male. The females can be used to produce more milk. But the males? Unless they are killed for meat, or simply culled to make space, all the grasslands of the earth will be rapidly overpopulated with superfluous male cattle. What would you propose to do with them?
There are experiments with mixing sperm to counteract this problem, but the only way forward, if a vegetarian diet is the default option, is in genetic modification, and that's a long way off.
So, it would have to be absolute veganism all the way. Good luck with persuading the whole world about that.