ActivityPub is just as susceptible to centralization as people claim Bluesky is with the only difference being that at least Bluesky has made some sort of mechanism for credible exit. It's old news that its expensive to run the perceived expensive parts, if fact you could do without it as many devs in the community are doing. ActivityPub is literally the easiest one to monetize because it can lead to moat building and user capture. It's why Meta is more than willing to become the biggest node in the Fediverse with Threads.