The whole attitude here is bizarre. It seems Persona is supposed to have taken over the world by now, while in reality it's not clear that it's even in beta, let alone out of it. E.g., we're supposed to get the javascript lib from one place only, because we're warned that the protocol could change in breaking ways at any time. Let's get to version 1.0 before complaining that facebook hasn't been driven out of business!
I support the recommendations to simplify and get rid of sessions.
So far, it seems to me that persona is an incredibly useful technology in our space (Enterprise SAAS apps). Our space is populated by lots of apps that have various shitty login systems, but one thing they pretty much all have in common is an email address - and Persona seems to be a killer SSO system for securely leveraging/livening up these existing databases of email addresses.
Unless I'm missing the point, I don't see much alternative either. Just looking at the traffic from the Persona communit reinforces to me that I never want to be building my own authentication system - even if persona were to die as a project I would be better off cloning it than writing my own (or giving in and using social login, which is pretty undesirable in the enterprise world).
Barring vandalism (which this isn't), you can think of it as a public internal wiki for Mozilla, that has pretty much everything we're allowed to make public (Unlike HR stuff for employees, which goes on an internal wiki) that we want to write down.
(This doesn't address your argument, just wanted to clarify that)