I think email evolved when people had more digital-identity ties to their ISPs though. I remember having an email address tied to your university or your dial-up provider or whatever being pretty common in the 1990s, and pre-eternal-September the same protocols already existed among a more closed crowd (which is kind of like Facebook, now that I think of it).

Then “free email” providers on the early consumer Internet were able to compete on things like storage, because that was something it was still acceptable to expect people to pay attention to. One of GMail's original big draws was “lots and lots of storage”.

Email being in decline for social purposes feels partly related to changing feedback expectations and UI inertia, but I'd guess also some relation to a “mental association with uncool things” including both spam and stressful/boring transactional email; it's become more of a business thing. The spam side of that is related to the gradual federation lockdown, and something similar could happen to Mastodon, but at least there's some discussion about it happening in advance now.

I'm not sure what this all adds up to.