I think the window had closed on Duke for a successful sequel anyway. Its no-holds-barred style, which seemed so groundbreaking at the time, long ago became a standard for shooters, and it's hard to imagine how a sequel would have much new to offer other than a simple nostalgia trip.
Don't get me wrong, I'll line up to buy DNF if it ever comes out. But I just have a hard time seeing it has a huge hit.
If it's a good game - why not?
Of course they'd probably need to start over again to get the game up-to-date to modern tastes.

I remember playing Duke3D on my school's LAN network during class - that was fun.