My first thought was “yeah but there are also things you can not undo like sending e-mail” but then I remembered there is a really nice undo send button now (that works by delaying the send a bit).
Sure there are things you can not undo, sometimes you do want to permanently delete something, but maybe with a bit of thinking a undo is feasible more often then one might think :-)
A delay which can be annoying in its own way. It’s rarely an issue to be fair but it means you may not be informed of errors if you leave the mail client too soon. Also there are (admittedly very rare) cases where you want the mail to be sent immediately but it is sent at an unpredictable time : those undo features rarely display a countdown.
Sure there are things you can not undo, sometimes you do want to permanently delete something, but maybe with a bit of thinking a undo is feasible more often then one might think :-)