I have no doubt that 4e D&D is the top dog with the lion's share of the market, and nobody else can get close. The problem is, most 4e players aren't buying 3PP 4e products for a variety of reasons from they can't use them with the DDI, to they have no respect for the quality of 3PPs based on their actual experiences or (more likely) misconceived perceptions based on the days of the d20/OGL glut.
It wouldn't surprise me at all if the majority of people buying 3PP products right now are doing so for something other than 4e.
Look at Adamant Entertainment - they aren't really releasing anything for 4e, d20 Modern, or True20 anymore. They've found Savage Worlds and Pathfinder.
Ultimately, 3PPs have to follow the money.