2) Your claim that people turning vegetarian would have any significant impact on global sustainability. The eco-problem of agriculture is not simply the destruction of landscape, but the loss of genetic diversity, which plantations can achieve on their own. Sure, you can replace monocultures with polycultures and get better biodiversity and higher yields, but these would be harder to manage. It's funny you say humans would manage agriculture rationally, as
this could be applied to animal farming too. In the end, no matter what we eat, it will follow the principles of contemporary economy. Everyone eating veggies means they will have to be more accessible. Fruits and vegetables may be cheaper than meat (though in my country some come close) per kg, but not by caloric value. If they grow cheaper and everybody buys them, producers will be interested in quantity over quality, which means treated crops (that's already a popular choice), and low costs, meaning they wouldn't care if the production process is eco-friendly or not since that would lead to supplimentary expenses.