CDI ignitions have made small gassers much more viable. I don't think there will ever be a commercially produced gas engine that ignites using a driven glow plug, because a CDI unit can do the job for a 1/3 ounce weight penalty. There's just no reason to do it.
But I do agree that gassers will continue to eat away at glow from the top. The first generation of .60 size gas engines have been on the market for a few years now, and they aren't bad. I expect they'll improve with time as glow engines did, so we very well could see .40-.60 size gassers that can match glow in the power to weight department within the next 5 years or so. They'll probably do it at lower RPM which means a new learning curve on prop selection, but they'll do it.