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

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.