I really think it's a mistake to assume that there will be one thing which replaces Twitter. This has ~never happened (I think possibly the only example would be the Digg->Reddit migration); normally people leave a dying social network for a _variety_ of other things, not for a copy of the dying thing.

In an ActivityPub world, this is particularly unlikely to happen. Arguably, we've actually had a social media ecosystem a bit like this before; there was a time when Wordpress (self-hosted or service), MovableType and a few others could interact via things like pingbacks (a sort of early quote-tweet).