In your example, if bluesky goes you lose it all (until/if their form of federation actually exists and is usable). If the Mastodon server I’m on (toot.community, because I like the name) goes, you move to another or host your own and keep on trucking.
When reddit did their API shit i left and lost the account, subreddits, apps, everything. When Lemm.ee announced it’s shutdown i moved to lemmy.zip and picked up exactly where I left off.
It’s not theirs to take over. There was barely any communication about this, they asked for admins a couple of times then posted about shutting down. They didn’t offer for someone else to host it.