The final problem is that not many people will write a full implementation of
   it other than the big players like mozilla itself or Google or Microsoft.
   And the latter two don't have a ton of incentive to do so.

When I see such a proliferation (I looked at one architecture image of OpenID.connect and was shocked) than I am reminded of SOAP and I again think, that one reason might be, to hold smaller players at bay -- and to give the bigger players an advantage. For Facebook, Amazon or an other big internet company, it is really easy to even implement a big stack of software with complicated architectures and many features -- but for a four people start-up it is a big problem.