On the other hand, those people can mostly run their own IdP, so they depend on fewer/more trustable parties (just the DNS registrar, and their colo?), but still get the same user experience.

Which I think is an important property: if you can come up with a better or more secure IdP experience, you can enter the ecosystem without the ecosystem (that is, Relying Parties) having to change anything. That's a huge win over the current state of login.