Generally it's good practice for AT apps to treat you as not signed up if you have not explicitly signed up to this app before. So I think a well-behaved site you never used shouldn't show your profile page on request. If this is right, maybe it needs to be more emphasized in docs, templates, etc. Same as they should correctly handle lifecycle events like account getting deactivated.

Realistically it's possible some services don't do this well but it's not clear to me that this is going to be a major problem. Like if it gets really bad, it seems like either this will get fixed or people will adjust expectations about how profile pages work.

I think what may be confusing you is that Bluesky (the company) acts in two different roles. There's hosting (PDS) and there's an app (bsky.app). You can think of these conceptually as two different services or companies.

Yes, when you sign up on Bluesky, you do get "Bluesky hosting" (PDS). But hosting doesn't know anything about apps. It's more like a Git repo under the hood.

Different apps (Bluesky app is one of them) can then aggregate data from your hosting (wherever it is) and show different projections of it.

Finally, no, if you're famous, you don't need enterprise hosting. Hosting a PDS can be extremely cheap (like $1/mo maybe)? PDS doesn't get traffic spikes on viral content because it's amortized by the app (which serves from its DB).