IIRC, it's nearly impossible to self-host email anymore, unless you have a long established domain already. Gmail will tend to mark you as spam if you're sending from a new domain. Since they dominate email, you're stuck with their rules. The only way to get on the good boy list is to host on Google Workspace or another established service like Protonmail.
That's on top of the fact that correctly configuring an email server has always been a PITA. More so if you want to avoid being a spam gateway.
Say everyone agrees and the entire world swaps to some alternative. Email 3.0 or whatever.
Wouldn't we just have the same issue? Any form of communication protocol (that can be self host able) will get abused by spam. Requiring a lot of extra work to manage.
Setting up a web of trust could cut out almost all spam. Of course, getting most people to manage their trust in a network is difficult, to say the least. The only other solution has been walled gardens like Facebook or Discord, and I don't have to tell anyone around here about the problems with those.
Isn't the current email system kind of a web of trust. Microsoft, Google etc.. trust each other. But little me and my home server is not part of that web of trust making my email server get blocked.
Realistically I don't see how it would ever not be managed by a corporation. Your average person doesn't know how and doesn't want to manage their own messaging system. They are just going to offload that responsibility to a corporation to do it for them. We are just going to have exactly the same system we have now. Just called some else besides email.
I wish there was a better solution but I am not seeing a way that doesn't just end up the same as email.