That wouldn't be Gmail in any sense. Why would google be involved in that product -- they dont' provide the server, the delivery, the spam detection (right?)
If you dont want to use Gmail for some or all messages, just don't use Gmail.
If you dont want to use Gmail for some or all messages, just don't use Gmail.
But I agree with you that it's very different from anything they do. Also, too vulnerable to piracy or cloning. Aaand it may not be feasible to run their bots in a weak machine.
Gmail ad targeting isn't a bit of JavaScript that can run in real time in the background. It's a series of huge map reduces touching data sets larger than any client computer could store. Just indexing a pre-prepared list of mothership ads would result in a horrible user experience (far poorer ad targeting).
Google has a vested interest in only showing you ads you might be interested in; it doesn't work as well as it should, but it works magnitudes better than the state of the art 10 years ago.
And if the protocol is sufficiently closed to prevent open-source clients, why would anyone use it at all? Wouldn't that be the same as the XMPP->Hangouts transition that infuriated everyone a few weeks ago?