Laugh all you like, but kids and the interns I've chatted with see email as something akin to how I feel about fax machines: not something they'd use by choice, but necessary for historical reasons. When I've been at organizations during a Slack adoption, email volume drops hugely; 50-80%, I'd guess. I've closed most of the mailing lists I used to run because they shifted to mediums better suited. Just this morning I had to FB message a few friends to get their current email addresses, which tells me a lot about who's winning.
Yes, I agree there are times you don't want things unsent. And there are times you do. There are a variety of ways to balance these concerns. But exactly none of them involves saying, "Lo it was handed down to us by Postel the Wise, and none shall tamper with His choices."
Asking to unsend an email is a reasonable request. The answer could be yes or no given the circumstances, but it's only an absurd question to people who have taken a 1980s technological choice and treat it as some sort of unalterable gospel. It reminds me of this Douglas Adams quote:
"I've come up with a set of rules that describe our reactions to technologies: 1. Anything that is in the world when you’re born is normal and ordinary and is just a natural part of the way the world works. 2. Anything that's invented between when you’re fifteen and thirty-five is new and exciting and revolutionary and you can probably get a career in it. 3. Anything invented after you're thirty-five is against the natural order of things."
Sure, I get that a lot of people were born after RFC 822, so it seems like the natural order. But the people who wrote the early RFCs weren't thinking that way, and neither should we.