Email does have an expectation of durable persistence; in fact I would argue that that is a critical feature of email - If you send me an email I can always return to my email inbox and if I open the email I will always see the exact same message. I do not want that to change, in fact, as soon as I know that can change, it undermines the trust of the entire channel.
Will Amazon send me an order reciept, then subtly change the delivery date because it loads via Javascript? Wow, I could have sworn they promised it would arrive on Friday, now it's next week? What else could they change in my order as well? If I want an immutable copy do I now have to go back to paper printouts?
Even worse, what if I got an email using interactive features from a website that no longer supports those resources? Am I going to open my inbox and get a 404 because it's dynamically loading content that now isn't there? I have a reasonable expectation that when I open an email today, I can return to my inbox and see exactly the same content next week, next month and five years from now without having to question my own sanity. If I need to look at an order confirmation from a website that now doesn't exist because they didn't get their funding am I SOL because the CDN serving the dynamic email content is gone?
This is why I oppose AMP. This is a serious, legit criticism and I'd like Google to address it head on. User's don't want emails to become ephemeral, ever changing media because they want to feel their inbox belongs to them. As soon as it feels like they're visiting a site that belongs to someone else, it's not their inbox, instead it's just a bunch of bookmarks to changing, volatile resources that someone else has put into their face. It feels like an invasion, and it is.