The use case you described is genuine. But the problem I see here is the insistence that the email platform should fulfill those requirements instead of creating a new platform and letting it win the market on its own merit.

People have certain expectations from emails, which have remained largely unchanged since the emergence of the internet. Those include a federated and fully open platform, immutability of messages that make it valuable as communication records, privacy afforded by plaintext, simplicity of use, etc. Many changes have already ruined some of those qualities of emails. For example, introduction of HTML in emails have converted emails from a messaging platform to an ad and tracking platform, forcing many clients to block dynamically loaded resources. Quoting of prior messages have become a complete mess. But worst of all, the email platform is arguably no longer fully federated, now that it's nearly impossible to self host email servers.

It wouldn't be a stretch to argue that changes like these are intended more to centralize the email network than to add features to it. AMP is a clear aggression in that step. It's telling that neither AMP for web, nor AMP for email survived once Google was forced to stop pushing the so aggressively. Makes you question who wanted it so badly and why.

The chance of another distributed platform with the properties desired seems small. Why should email be resistant to any change? Takes like this is why every company develops their platforms as silos rather than open standards. To avoid the inability to ever make a change once it becomes popular. Instead, at best, we end up with large monocultures around an open source project such as Linux or chromium. Maybe that's better and email like platforms are a mistake, but fundamentally I don't feel like that should be true.