There's no lock in, but migrating away from Persona isn't quite that simple. You still have to educate users about the switch ("make a new password, and by the way, don't use your Persona password!"), then implement everything Persona provides: login form, password change form, email authentication, ...
If Mozilla did migrate away from persona, would the staff become less familiar with the details over time thus making the best effort support being offered by the community in sumo and being talked about here: https://support.mozilla.org/en-US/forums/contributors/710099
My question for you all is what do you think could be supported once it has been depreciated?
