Ok, maybe nobody cares about 3D Realms, but plenty of people want a sequel to DN3D.
I think the problem with DNF is the IP holder has a "Howard Hughes" or "Rain Man" style tick, with obsessive and unachievable perfection standards. Everytime the game engine or some other aspect gets so far, another game comes out with a better engine and they must just start again.
The "When It's Ready" mantra is not just wearing thin, it's worn through. Decide on an engine, get the game out, sell lots of copies and start work on another sequel. In the time they have spent trying to release the best game possible they could have released 4 or 5 sequels. The "Duke" image is strong enough to support an almost limitless franchise, but you have to have the game out there in the first place to be a franchise.