I'm always surprised to hear people say "The world is overpopulated" - I'm thinking there's plenty of room to grow in regions that live sustainably.
Then there is the argument that glasshouses are 10x as productive as open fields, so if we were to use them predominantly, we could return land to forests.
Quick google-fu is failing me with actual numbers, but I would not be surprised if we are using 3/4/5 orders of magnitude of water every year compared to 3000 years ago.
the environmental benefits would be enormous
That's all before you even talk about greenhouse gasses.