A lot of people do not want to wear a pair of glasses all the time. The downside they have is you have to put them on to wear them. A screen on the other hand you can glance at without the physical act of putting it on first. I don't think glasses will replace the phone but instead they will augment it. The phone also makes sense to use as a processing computer unit tethered to the glasses, which can reduce the weight of the glasses. With this setup you get all things.