Hussar
Legend
I don't remotely believe that too many supplements hurt sales.
I do believe that poor quality supplements and/or supplements that don't consider poor interactions add a non-zero negative weight on the gaming community perception of a system.
Time is the real enemy.
It is easy to look at the increasing stack of supplements and blame that for the concurrent decline caused by age.
What age? All three previous WotC editions died after 2 or 3 years. 5 years if we're being generous.
That's not much time between reboots.