There are also regular surveys for most things. WotC wants to know what its audience wants to better serve that audience. I feel like a lot of people forget that when they wonder why WotC does or doesn't do X, Y or Z.Seem that in therm of design Simplicity take over Modularity.
They choose to step down on new classes like Warlord and Psion for some reasons, but not because the system was not modular enough. There are other design and product philosophy that govern those decision. The gradual increment is chosen IMO.