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.
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?
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.