I agree that 5% to the engine developer is a wonderful deal - however, having a working game engine in no way means 90% of the work is done. That's when the really difficult work begins.

Video games need to trigger an emotional response in order to be successful. From visual design, to audio design, game rules/logic, controls, testing, marketing...this list goes on. It's far more than raw tech resources; psychology is a huge factor in game development.