Argyle King
Legend
While I agree with your argument, one could also argue that the lack of new ideas or synergies with previously existing classes indicate that maybe they weren't worth adding in the first place.
Look at 3.5 as an example. The Complete books added 12 new classes to the game. I think it became obvious from the books that followed that some of the classes (warlock!) had really taken hold in people's imagination and some others (samurai!) didn't.
Personally, I liked the Samurai; however, it was quite poor in a mechanical sense. In a system where disparity between classes already existed, it was difficult to justify choosing a class which was arguably less useful than the fighter.
The warlock got support because it was an early test of 4E material. It was a popular class and I do not doubt that had a hand in garnering more support, but I also suspect it got an added boost in material due to the fact that it was already known to be part of the next edition of the game.