You could always use IMAP to sync your email as normal. The advantage of IM2000 is that it just changes the server-to-server protocol (with some changes for how clients send emails).
But yes, the transition would require replacing SMTP so once again we're stuck on a system that is terrible enough to grumble about but not terrible enough to change.
(I would also argue that Matrix is becoming a new decentralised system for the internet, as well as ActivityPub depending on who you ask.)
There also isn’t an easy transition from now — I can see exactly why we’re “stuck” with what’s actually a pretty great last bastion of decentralization on the Internet, for all its warts.