There is, it's called IM2000 where email becomes a pull-based system instead of push-based (so users would have to consent to spam, and spammers have to store their spam to send it in bulk). Unfortunately, despite many attempts to switch over we're still stuck on the current incarnation of SMTP.
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.
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.)