That is also a reasonable request.
I'm unable to fathom the notion that if a computer doesn't work the way people want, the answer is for people to adapt to the computer. The whole point of computers is to do things for people.
With physical messages, unsending has at least partial support. Before the mailman picks up from my porch, I can grab a sent message any time. If you FedEx the envelope, you can cancel before delivery. With a university's mail system, you can get the receiving department's admin to return something even later in the chain. And of course, you can always tell a recipient, "Hey, I sent you the wrong box, just send that back."
The reason email doesn't support unsending is not some essential property of messaging. It's just that at the time our email protocol was defined, both our hardware and software was pretty primitive, so we locked in a very primitive model of messaging. But note that more modern systems, like Slack and Facebook Messenger, happily let you unsend things. And consequently, they're effectively replacing email for most users.