Just listened to 38, on the topic of "why release a new version of dnd?", which came around to a discussion point suggesting a ruleset can be separate from setting, and settings can be developed independently - I agree, and I had a thought:

I think there is a point where players wish to pick-up and play a game without re-work or development. Paying for a bundle of setting + ruleset is time efficient and cheap.

There is also some settings which have proprietary mechanics in the rules (swarms in DeathWatch, flexible magic in Arsmagica) which would be difficult to emulate in other systems. eg. implementing either examples in GURPS would be interesting and probably fun, but would also take significant time to create, and then more time for the other players to learn. With a single book game the players start playing with minimal run-up.

My point is moderately diluted by dnd due to so many editions that are so similar (1,2,3,3.5), except that the choice of mechanics is sometimes very important to players (as seen by pathfinder's initial uptake), and the separation of dnd4e into what is essentially a new game with dnd branding.

Just a few thoughts, great podcast.

Thanks for your feedback !