Google Glass will never become mainstream. Who is going to go around wearing a pair of these? Societal perceptions matter, now whether that is a good or bad thing may be up for debate, but it does.
Now, a contact lens, yes, that is a breakthrough product that appeals to everyone. Who knows how far away from that we are though.. Probably years and years.
it doesn't even have to be something as unobtrusive as contact lenses.
in a few years miniaturization will likely make it possible to make Google Glasses which would be indistinguishable from regular glasses in appearance. that would change things in terms of public acceptance. I still think this is the future. It's just that Google jumped on this years before the tech was ready. They like doing such futuristic things. they create a lot of buzz but don't go anywhere commercially.