Have you asked people if they want it? Any time I make a mistake I think about the ability to unsend or edit what was already sent (within a time limit etc etc).
Generally speaking, people aren't going to think, let alone talk to others about a feature that doesn't exist in most platforms they use.
But clearly people do want the ability since Exchange provides that feature, and GMail will delay sending your message for up to minutes in case you want to "Undo" the send.
The gmail case isn't an "unsend" feature, it's just a delayed send.
OP seems to be suggesting you can take back a message once it's sent. This opens up issues since the receiver may have already read the message. Now you get into cases where someone says "Bill threatened me over Facebook messenger" but now Bill has deleted that message.
Generally speaking, people aren't going to think, let alone talk to others about a feature that doesn't exist in most platforms they use.
But clearly people do want the ability since Exchange provides that feature, and GMail will delay sending your message for up to minutes in case you want to "Undo" the send.