Actually, you're wrong. Zaplets made this work somehow, and were funded to the tune of $90+ million in VC money. Perhaps they just used an iframe or , but never the less, Zaplets offered interactive HTML forms inside of mail that worked in Outlook. (
https://www.forbes.com/forbes/2000/0612/6514218a.html#224241...) AMP Email is largely following what the Outlook 97 client was capable of.
There's no apriori reason why a federated messaging system needs to be non-interactive. It's possible to support both immutability and dynamism. You're making assertions that something should be a certain way without actually justifying it.
You could at least provide some formal justification, like you want Email to be an immutable ledger or something. But the idea that Email can't evolve and change and should stay exactly the way it was, pretty much assures its death in the dust bin of history, the same way we lost USENET, LISTSERV, or even XMPP. Old standards didn't evolve quickly, users wanted silly features while neckbeards said don't violate purity of my abstraction.
And today, Facebook, Instagram, WhatsApp, WeChat, iMessage, et al, are the predominant messaging platforms, and email is for spam, receipts, and grand parents.