What would make it a bad idea? Just delay sending it for a user-specified amount of time (I think Gmail does 5s, 10s, and 30s), then have the UI for undo-ing th

What would make it a bad idea? Just delay sending it for a user-specified amount of time (I think Gmail does 5s, 10s, and 30s), then have the UI for undo-ing the send disappear after the time elapses