I think you have some good points, at the same time, here where I live, in the food store, a part of the freezer is filled with plant based food - if people hadn't voluntarily decided to be vegetarians, then, there would have been meat instead.

When enough people change their behavior, that can have an effect. And I suppose you're right that that effect is smaller than what one would hope (for the reasons you mentioned), and laws or taxes would have more effect