At least the long-forgotten Descent to Undermountain was built on a modified Descent engine.
Freespace and Descent had nothing in common besides a dev team, and even then Parallax Software had already split into Volition (who handled Freespace) and Outrage (who did Descent 3 and collapsed after one or two more games). This was one of those dumb Interplay marketing decisions that I think may have hurt the franchise in the long run.
