We need a modern design for a successor protocol to email, and no one is working on it because they prefer instant messaging (or think other people do).
Everything would have to support SMTP as a fallback for the lot of people that just don't care and thus couldn't actually improve.
One day, a while after they become usable and common, people will just realize they've been sharing documents E2EE in place of sending email, and they'll be using it for basically everything that matters.
It would be a proper restart and allow for significant improvements in usability and security and everything else.