My assumption is that the fediverse is seek to replicate what was already possible in the centralized manner---a large-scale social network in this case. Mastodon is of course decentralized by nature, but but it is not yet a decentralized social network under my assumption because it doesn't fully demonstrate a large-scale social network right now. To be clear, neither does ATProto/BlueSky right now! But scaling a social network is shown to be hard even without decentralization and it seems more likely that Mastodon would hit the scaling issue earlier than ATProto. That's what I meant by "capable for eventual decentralization".
One particularly visible weakness of Mastodon is that there is no working and efficient mechanism to operate on the entire network. Searching and algorithmic feed [1] would be major features affected by this weakness. I know some consider this to be intentional, but if that's true then Mastodon can't be really said to replicate a social network at all. It is yet another matter whether such features are warranted or not, but if we believe that something like a worldwide social network should exist, then we don't yet have a clear answer that a social network without such features would be technically or socially scalable to that extent, so not being able to fully replicate an existing social network is important enough to consider.
[1] This includes trivial feeds like all posts from my followers, because in my knowledge a large number of follows is not really scalable in Mastodon.
One particularly visible weakness of Mastodon is that there is no working and efficient mechanism to operate on the entire network. Searching and algorithmic feed [1] would be major features affected by this weakness. I know some consider this to be intentional, but if that's true then Mastodon can't be really said to replicate a social network at all. It is yet another matter whether such features are warranted or not, but if we believe that something like a worldwide social network should exist, then we don't yet have a clear answer that a social network without such features would be technically or socially scalable to that extent, so not being able to fully replicate an existing social network is important enough to consider.
[1] This includes trivial feeds like all posts from my followers, because in my knowledge a large number of follows is not really scalable in Mastodon.