There's a good chance that the ecosystems evolving around ActivityPub, WebFinger, may parallel mail (smtp, pop).
An era of innovation & cooperation.
Then API battles as some scramble for control (vim, mapi).
Then the community digs in to find ways to deal with the problems that come with scale (spam, security).
Settling into the duopoly of A (gmail) vs B (o365), and "other" (fastmail, ...).
The scale phase will be much more difficult in this case, though, but it wouldn't be a bad outcome.