The vaccination situation proves that there are idiots in the world, if you take those people as example of "can't prove X" then we might as well stop all scientific research right now. There's also people who believe the world is only a few thousand years old.

To get societal (and legal) change, you don't need to convince 100% of the crazies. Convincing most of the sane people is enough.

(I'm not saying only crazies are against psychedelics, just that the vaccination comparison isn't useful.)