I think BlueSky is the bigger threat, to be honest.
If the fediverse population ‘clumps’ into mastodon.social and mastodon.social sells out, people will still be on the protocol so they’ll be able to transfer out.
But if BlueSky succeeds at replacing twitter and filling up the niche, less dedicated fediverse users will switch to the livelier site. That will be a far more devastating blow to the fediverse.
That said, I think it’ll survive either or both. ActivityPub taking over twitter instead of BlueSky would be good for the open, anti-monopoly internet cause, but I think it’ll continue regardless. The core communities are established.
I think the fediverse should replace popular socmedia, but it will never be able to compete financially.
We’ve already got Bluesky, which is the same thing but controlled (and sponsored) by the usual suspects, poised to snap up any users that bail from twitter. And popular opinion favours Bluesky thanks to the positive coverage it gets compared to fediverse projects.
The fediverse in the form it’s in now will never replace twitter while the free market controls the distribution of users. They’ll always go to the places controlled by big money.