I'm not sure what the long-term plan will end up being. I personally hope it can find a new direction to grow eventually. (Note: I'm a Mozilla developer but I don't work on Persona or Firefox Accounts.) We use Persona extensively for our own sites and tools, so I'm sure there would be some sort of migration process if Persona were ever shut down in favor of some replacement.

For a more informed answer, you can ask the team directly in the mailing list or IRC channels listed at the bottom of https://developer.mozilla.org/en-US/Persona#Join_the_Identit...

[Edit: Or see callahad's reply in this thread.]