That's a cool tech. If it really provides decent attenuation of WiFi without affecting other radios I think it may still take off in the future. There will be a point when 2.4GHz WiFi doesn't work anymore at all.

How do you get to this conclusion? The density is probably not going to get much higher than it is already, but the technology is going to continue to improve.

2.4GHz technology has already stopped improving. It's just getting more crowded as "the Internet of Things" trend gets more popular, flooding the 2.4GHz band with even more devices that—owing to their low-power design—often support only 802.11b or g and only at low rates, making them horrifically wasteful of airtime.

More high-def everything, people moving from cable TV to streaming, WiFi displays, ever growing software updates, realtime online games... Growing expectations may easily compensate for technological improvement.

Maybe wallpapers are too much pita, but if it came preinstalled in the walls and floors? Why not?

Do the wallpapers/paints block cell signals at all?

They can if they can form a faraday cage