I don't believe that Google Glass will ever go mainstream. It's just a niche product for geeks. it will take a very very long time before they show up here in Europe. If ever, I think it will die out before that even happens.
The reason is that a product you have to wear as glasses, for persons that doesn't need normal glasses will always feel awkward to wear. Personally I don't want to wear glasses, not even if I have to because of bad sight. If I have to when I get older, I think I would choose lenses instead.
I can't see that all people will wear glasses in the future. I simply can't.
I believe wearable tech need to be something else. I'm not saying that smartwatches is the way to go, but personally I would wear a watch over glasses any day.