Bluesky is decentralized, its just decentralized in a different way than Fediverse apps. A lot of fediverse people assume all decentralization would look like ActivityPub, so they just say its fake decentralization rather than learning how it works. (I know because I used to do the same, then I realized how little I understood it from this great blog post, and have since learned a lot more about it.)

There is already alternative infrastructure available (i.e. Blacksky and a variety of other applications hosted using ATProto (you can see a few here: https://bsky.social/about).

You can use any of these apps while maintaining full control of your own data by running your own PDS, or using any community maintained PDS. If you already have an account on the Bluesky PDS, you can migrate it, retaining all of your data. If you dont feel like migrating yet, you can also just export your rotation key, which would allow you to maintain control of your account even in the event that the Bluesky PDS does become evil or something.

Speaking of ATProto, sprk.so is a similar upcoming app, although I think its going to be more similar to TikTok than Vine.

Yes.

It is actually decentralised as far as I can tell, although I would like to point out that bluesky is also decentralised, while bsky.app has dominant market share, nothing stops anyone from hosting their own, completely independent, instance. Some have already done so.

Nostr is part of the fediverse, as is blueksy's ATProto. Neither Mastodon/LEmmy (ActivityPub) nor Bluesky (ATproto) are decentralized. They are poly-centric, and calling Bluesky poly-centric is still a little bit of a fib. NOSTR is truly decentralized. Here on Lemmy if the admin of piefed.world gets mad at the admin of lemmy.ca we couldnt' communicate anymore unless one of us got a new account. On NOSTR if the relay owner no longer wanted my content, you can connect to a different relay that does. (For example)