Here is EN World’s take on what the cool kids are playing at the moment, or at least talking about. If you prefer pie charts, there’s one here. I can stop doing these now EN World has automated the process, using a much larger sample size.
…and here’s what it looked like in 1999, the WotC Study that led to the OGL, as it could be interpreted to show that all gaming activity drove up sales of the D&D Player’s Handbook.
All of which tells you that the hobby consists of D&D plus debris, although what used to be D&D is now split into “official” D&D and Pathfinder.