Yeah. I wonder if it helps to look at RPGs that are on the lower line of success. We know Shadowdark is successful.Exactly. I think we can all agree that games like Pathfinder and Call of Cthulhu are successful RPGs.
Time also changes success. Is the original 13th Age successful now? I'd argue it was successful in the years after it came out. I played it at conventions. They had a lot of great first and third party support. But I don't know how many people are playing it now. Of course, that could change when rev 2 comes out.
Shadow of the Demon Lord is in a similar boat. It still has lots of first party support but are people really playing it with Weird Wizard out now?
I heard Fred Hicks talk about how interest FATE had died off, not just as a game but as a whole system, given the rise of Powered by the Apocalypse.
So yeah, time has a factor too. Games that remain successful for a long time are pretty interesting to see.