Email was invented in 1965. It needs to be replaced with something that doesn't try to bolt security on as a lazy afterthought 30-40 years later.
We have everything we need to define email 2.0 with mandatory security, the problem is all the older installations that support 1.0 that everyone still expects to talk to.
New protocols only solved this by cutting compatibility. Signal can be E2EE because you can't send a message to an email address with it. Email can also be E2EE, but how do you know the recipient supports it?
It shouldn't BE backwards compatible. That's the Microsoft problem. They can't innovate because they have 40 years of legacy holding them back. Email might be the only situation that's worse. I don't want another new layer over the top like DMARC, and I don't want to try to "fix it." It can't be fixed, it's just a bad design.
I want something fundamentally new that doesn't stink of 70's era pre-security thinking.
A group of heavy hitters needs to just form a working group and propose Email 2.0. As long as it's supported by the biggest vendors and companies and they don't go all "IPv6: It's better because it's complicated!" on it people will sign up because it will be able to offer MUCH more in terms of features, usability, security, ease of hosting, etc.
Sorry to break it to you, but email isn't going anywhere anytime soon.