One day, I really do hope there's a way I can take my email address and leave Google. Number portability deserves an analog in the email world.
It has one; register your own domain, then you can move it to any registrar and provider you like.
The analog going the other way would be if you took a Google desk phone number where people had to dial Google's offices into their phone system and then dial your extension, and you wanted to take your (Google office + extension) number elsewhere. That kind of service doesn't exist in the phone number portability world either.