I don't really see the point in this and this is why: let's say Persona won and all the big ones switched to it (Facebook, Google, Twitter etc.) by becoming pro

I don't really see the point in this and this is why: let's say Persona won and all the big ones switched to it (Facebook, Google, Twitter etc.) by becoming providers. Most people would still be using their Facebook, Google or Twitter persona anyway (except for a few privacy sensitive users who don't like SaaS anyway because it's bad for privacy according to them). So we would have a situation that would be de facto very similar to what we have now (3 sigin buttons for facebook, google and twitter) and then an extra one for people using their own hosted provider. This is what happened with OpenID, problem was that supporting people using their own providers became a nightmare as these providers went down or had some incompatibilities. People were just angry they couldn't sign in with their old providers, many of theme may have not even known they were using custom providers or what providers are and the other half would be privacy nerds who enjoy complaining every time their custom provider stops being fully compatible. For app owners, it would just be a big waste of time and resources supporting these users.