I totally agree about the large instance problem on fedi. One way to look at Bluesky is as a very large instance with internal structure attempting to avoid moderation and monoculture problems, tbd how well it succeeds on that front. And Bluesky PBC currently is far more dominant in the ATmosphere than Mastodon gGmbH is in the fediverse, so the consequences of somebody buying it are more significant than somebody buying Mastodon gGmbH (which could certainly happen although it doesn't strike me as particularly likely).
In terms of data portability, if somebody gets kicked off of .social they lose their posting history and social graph unless they've backed it up. If they have backed it up (or if they migrate before getting kicked off) they can largely recreate their social graph someplace else (although .social can still block them from communicating with anybody on .social, and there might be other instance-blocking issues, plus migraton often loses some followers). In principle the Bluesky situation is somewhat better: you can import your posting history into a new PDS, and if you're running your own PDS you don't need even the separate import/export step. In practice though right now there isn't a meaningful "someplace else" (and it's not clear if and when that will change) so the reality's not as good -- if you're blocked from the Bluesky AppView you basically can't communicate with anybody.
That said, Bluesky's story is much better story than fedi's "we didn't design for this and haven't made any progress in the last N years but since BLuesky started giving us a hard time about it there's now a SWICG working group that's got a draft spec that nobody's implementing!"
It's frustrating because this absolutely could be solved in an AP framework but nobody with resources has any incentive to solve it. In Cory's piece on Bluesky he said something like "Any system where users can leave without pain is a system whose owners have high switching costs and whose users have none", hmm I wonder why Mastodon gGmbH hasn't tried to reduce the pain of people being able to move?