It will stop being invite only eventually, and I don't think Google is going to kill the project it's along term play that they are investing a lot in, and as integrates with all of Google's other services it'll become more valuable and get exposed to the millions using those services.
Of course it will. It's a bottleneck, and once it's unrestricted and there is no friction to joining they will market it to the non-techies. And regardless even a techie exclusive user-base is far from worthless.
