The problem is how that intersects with SEO incentives. Basically, the platforms are expert at creating prisoner dilemmas such that everyone has to do work to help them instead of coordinate and pick what's best.
If I *have* eg. to use schema.org to rank, then I'll do that even if I don't actually want the rich link. It's defection for survival.
But I agree that licensing metadata is interesting, there's stuff around rights reservation metadata that could be promising.