Fine to imagine if you believe beans, carrots, onions, and cabbage can be grown anywhere.In my country brown, white and red beans are cheap if you buy them in larger pots. If you mix them with an egg, spices and breadcrumb you can make tasty burgers, cheaper than any meat. They have lots of proteins.
Fresh and cheap vegetables are carrots, onions and cabbage.
For all its inefficiencies and loss, it is more productive to grow grass on my crap soils and raise cattle on that than it is to try and grow feed crops directly.
And we won't even get into the math of what it would require in acreage to support current populations on a vegan diet, or the transportation costs associated with the increased mass and bulk to bring those foods from productive agricultural areas to population dense metro standa areas wih millions (or tens of millions) of population.
Humans are adapted to be meat-favoring omnivores, and the extra time we get not spent eating low value high volume foods as result has allowed us to do many great and terrible things.
There is no perfect solution, only trade-offs of greater or lesser acceptability.