It's already too late. It should have been fixed before Twitter nose-dived. Coming up with a replacement at this point isn't going to work: the Mastodon name is already out there, and this is the time when an alternative has to be ready and usable: it's first-mover advantage.
>There is a clear path to adding things like DKIM or SPF. First you...
So why hasn't this been done yet? We've had decades.
>The problem with email is that the major providers don't want to do more things like that
Exactly my point. Email *can't* be fixed at this point because of first-mover advantage. Incumbents don't want to fix it. Users moved to email because it was available when they were ready, and it was the best alternative at the time.
Mastodon is likely in the same place now. Something better doesn't exist yet, and doesn't have any name recognition. Mastodon itself can't be fixed because the designers like it how it is: if they wanted a better design, they would have designed it that way from the outset. So if Mastodon catches on, we're stuck with it as-is.