You can host your own instance, but resolving forks is not self-authenticating and requires some central trust (because of the 72 hour rollback window for higher priority rotation keys). Not counting that, you could essentially run your own fully independent instance where the worst that could happen is that you lack some newer updates to people's did documents (but anyone can upload them since they're self-authenticating). Some people do run their own instances for caching reasons, but these just ingest operations from the official one.

In terms of "credible exit", if the community at large could decide to move to a different PLC host, it would be technically possible for everyone to switch over.

Worth mentioning that Bluesky PBC is relinquishing legal control over the PLC and spinning it off into its own entity based in Switzerland.[1]

[1] https://docs.bsky.app/blog/plc-directory-org

No, did:plc is centralised, not federated or anything. The whole ecosystem relies on a server at Blue Sky PBC