Well let's be honest, the current 747-8 is hardly a commercial success with <100 in operation, mostly freighters, so why would Boeing even consider stretching the existing design's upper deck even more?
I always thought that Boeing never really believed in the projected market for an A380 size aircraft and was therefore betting more on the thinner point-to-point (Jacksonville to Novosibirsk if you will
) kind of routes, instead of the big hub-and-spoke trunk routes requiring massive volumes like the A380.