Is that not what reversible means?
In some sense, yes. I guess it depends on how you want to define your words.
Strictly, the action you start when you click `Send' is reversible for a few seconds, before it actually launches the nukes. But I'd rather call it `delayed' when talking to a human about it.
Delaying is a way to make some non-reversible actions sort-of reversible.
Nah, people just click through. GMail has a much better model: mask destructive / un-undoable behaviour behind a short timer, make the action undo-able until the timer is up.