Part of the problem with 2.4 GHz in big buildings is that it passes relatively well through building materials, so that in a large building you get a lot of noise from equipment owned by other residents.

Someone measured the difference in transmissivity for different materials at 2 and 5 GHz (http://www.ko4bb.com/Manuals/05)_GPS_Timing/E10589_Propagati...) - the difference is quite large for brick and cinder block; unfortunately, they didn't measure insulation foil, which I expect would be quite a good RF shield, especially if earthed.

So if everyone switched to 5 GHz, the interference situation might still be better than if everyone was on 2.4 GHz.

Of course, being an early adopter of 5GHz probably helps with the other problem in apartment buildings, the 'prisoners dilemma' type problem where everyone who knows how ups their transmit power because of the noise, making the problem worse for everyone else but temporarily better for them, when if everyone lowered their power, everyone would get better speeds (exactly analogous to how everyone ends up shouting when having a conversation with the person next to them at a party, when if everyone talked quietly instead communication would be more successful).