You can buy an electric setup for the price of a glow setup, there isn't much of a difference. The electric setup is by far cheaper in the long run, even if you have to replace batteries, speed controllers and motors once in a while.
Glow powered helicopters are rare.
I'd say we are already into that future. Glow engines are not economical, there is still a slight advantage of power / weight ratio you could take as a reason. But for me it is rewarding to get these little machines to run and I like the noise.
Glow engines will be around for some time, but we are probably the last generation to use them. But we'll hold on to this as long as we can :-)
Heck, if I can afford it I'll still drive my V8s when everybody has an electric car.