It's really easy to say "WOW THOSE BASTARDS R LAZY", but look at the size of the two studios involved.
3,500 man-months over 10 years is about 30 people, which fits with the 35-employee status on the studio's wikipedia page. Gearbox, by comparison, has about 170 people (again, according to wikipedia). This is a huge difference and, strictly speaking, if 3d Realms DID have the larger staff, the game very well could have come out in 2-3 years.
I think DNF was delayed so long less because 3d Realms is full of useless tools, and more because they tried to bite off much, much more than a studio their size could realistically chew, but never had the sense to spit it out and try to eat something more sensible.