DNF was more like a series of half-finished games, all worked on for a couple of years and thrown away. Nothing they showed the public ever had more than a year or two between it and one of the many restarts.

The analogy to a 20% project is actually pretty apt. 3D Realms was mainly a publisher. The cash from that business let them screw around on DNF almost indefinitely without worrying whether it would ever ship or make a profit.