But that's the thing: now you haven't made an email replacement, you've made email more complicated.
If you're backwards compatible you need a very compelling feature to get people to switch, and using the feature is limited until people switch.
That's how we added spf, dkim, dmarc and starttls.
The biggest difficulty is managing the layers of compatibility.
You'll have to walk me through what you mean by that. I'm not sure how what I'm hearing you say would improve anything.