How much more expensive would such a diet be? Some plant-based foods can cost a lot depending on place and logistics.
That's the question, and it's an important one - it's the same reason as why, say, WWW which started decentralized has become what it is now for most people. People do what's easier and cheaper for them.
I'm not calling people "soy boys", but people for whom you have to make such a diet not even plausible, but cheaper or as cheap as the existing ones, do not live in developed countries. Talk about the environment is not worth anything else for them.
Well, yes, this requires production of similar scale at every stage of the chain. With animals it's animal food, drugs, various machinery in production, etc, which also cost less due to scale on which they are produced.
Green alternatives in this case have a potential be honestly (without subsidies or anything) cheaper in the end.