I feel like fast food is a pretty big market for stuff like this. Burger King in New Zealand has had plant based alternatives to the whopper and chicken burger on the menu for > 1 year now so it must be doing ok. I'm not even vegetarian and I get them sometimes, they're pretty good (especially the chicken one - they changed the recipe a while ago and it's now practically indistinguishable from the real chicken option, although that probably says more about their standard chicken than it does about the meat free option).

There's no premium for the plant based versions I don't think (or if there is it's small enough that I never noticed), and I think you're underestimating how many vegans/vegetarians still want junk food.