Yeah, I've been thinking about this from a business standpoint ever since my earlier Conan suggestion, and I've pretty much come to the same general conclusion.
The problem, as I see it, is that most games and modules probably don't rake in tons of money. They have a somewhat limited print run compared to national bestseller books, they have a very limited sales outlet selection with only a few stores and online sales to work from, and I suspect the overall profit from most games isn't that large.
Then you have to factor in the merchandising fees. WotC gave up on the Dune franchise because it was too expensive, not because they didn't think there would be a market for it.
I suspect that whatever profit one could earn from a Conan game (or similar big-name property) would mostly get eaten up by the IP owners in the form of the license. I also agree with your point that having Mongoose produce so many Conan products would deminish the value of the license right now.
I was just thinking that a new RPG would be kind of neat if combined with a popular "name brand", but it could be that the DCC name is already established enough to sell the game by itself.