All of the alternatives that can actually happen involve handing over full control of the system to one company, and thereby eliminating email's major remaining value, which is that it is a thing not controlled by one company.

The technical problems are challenging but solvable. The human problems are not. Nobody with the resources to truly solve the problem is willing to do it to create an open platform where they don't get effectively all the money and control, but then, the rest of the world is not terribly willing to let them have all the money.

Hence the impasse we are at.

This is the problem you have to solve, not a technical one.

(See also "why the metaverse where we do things like share avatars across all services is a stupid idea that will effectively never happen". It writes well in a novel, but in practice it requires World of Warcraft to accept that someone can run around in it as Mickey Mouse while wielding a Call-of-Duty-branded sniper rifle, and none of the relevant rights holders will ever agree to that, for all kinds of reasons. The technical problems are also formidable but they are nothing next to the fact nobody will ever agree to this.)