I don't think that's even desirable and also legally questionable. But anyways, these posts are not gone with an instance shutting down and thus I don't really see a problem. You can always add a link to a mirrow of those old posts in your profile.
Content is mirrored on all federated instances and it is very rare for an instance to shut down without notice.
It's super easy to migrate accounts on Mastodon. Even works fine to move an account from Mastodon to Akkoma for example.
Mbin is a fork that was startend to add some stuff the original Kbin dev didn't like. These days it seems Kbin is dead though, so basically Mbin is the new Kbin.
Lemmy.world used to cost that much, but I think they downscaled a bit recently, or are at least planning to as the current growth of the userbase has slowed down.
I was actually surprised by that 150 figure when I first read it, as it is much cheaper than what the BlueSky documentation makes it sound.
It is certainly possible to collect that much in monthly donations, but then again... how do you build a loyal base of supporters for running a mostly hidden piece of infrastructure? People always complain about the instance focussed nature of the fediverse, but the ability to build communities around them and get people actually emotionally invested in their home instance is IMHO rather a strength of it. That is also why I am slightly sceptic of easy account migration tools, as it devalues the instance as yourhome base to a certain extend.
There are some people hosting their own identity server, but yes the centralisation of the main aggregator server seems to be by design as they even scare people away from trying by talking about the high resource requirements of doing so.
IMHO Bluesky is only federated in the sense that responsibility for content and moderation can be outsourced, but the user endpoint stays mostly in control of Bluesky. This makes a lot of sense if you think about it from a company perspective... outsource the legally and personnel critical parts and keep the ones that are lucrarive for advertisement and can be easily scaled by throwing hardware at it.
But you must be a real sucker to take them up on that very one sided offer...
Bluesky is explicitly promoting their system as "choose your own censorship" kind of deal, which in the way it is framed could look very attractive to right-wingers looking for an alternative platform. While this is technically also true for the Fediverse, it isn't promoted as such, and rather has a reputation for the opposite, as most fedi server admins are center-left leaning.
Bluesky might be also more left-leaning right now as obviously there is little reason for right-wingers elsewhere to switch away from Shitter to another (mostly) centralised platform, but given the overall low user numbers this could switch very quickly.
I guess we will have to see how this develops over time and get some answers from Brazilians that have a deeper understanding of the current social dynamics there.
Due to the language divide it might end up as two distinct social spheres, like Fedi's Japanese bubble, but that's a best case scenario for Bluesky I guess.
It would be interesting to know more about the additional users. The banning of Shitter in Brazil is very much tied to internal politics and AFAIK it might be the Brazilian equivalent to MAGA that is currently mass-migrating to Bluesky.
If so, this might be a bad thing for them, as they probably don't want to get perceived as the Brazilian truth.social.
This is kinda the same idea but made for what you originally asked for: https://garagehq.deuxfleurs.fr/
"Everyone"? I know exactly zero people that have admitted to having a Bluesky account, but I know plenty that have an Mastodon etc. one.
I think this is at most a very regional or specific user bubble thing, and official user numbers for these commercial services are never trustworthy.
XMPP via its pubsub implementation could function similar to ActivityPub, but that part of XMPP is a bit neglected as few apps use it for anything more complex. XMPP's real strength is in real-time communication and I think it's good to stick with that.
Can you do that with a letter once it is send? And the instance admin of the mirroring server can delete posts if that is legally required for some reason.
And how would that even work technically? Bulk import posts and spam other instances with mass updates? That would immediately detected as a spam-wave and blocked. And back dating technically new messages is also not exactly a great thing to allow.
Other implementations of nomadic identity like Hubzilla get around this by letting you run two accounts in parallel and syncing them from your main account, but they will also not back-port old messages before you linked up the secondary account.
Basically anyone with some experience with federated systems agrees that importing old messages in bulk on account migration will never happen, and I don't really see an issue with that, since messages are not lost.