100% this. I did a deep dive into Gen Con events, looking at them over a six or seven year period. I'll try to remember to dig it up tomorrow and post some interesting tidbits.
When I worked on D&D, the core challenge was getting people to play the new thing. If we did an adventure or a sourcebook, our goal was to design and market it in a way that got DMs and players to pick it up and start using it. If you play a thing, by definition you need to recruit 2 to 5 other people to take part. That drives so much growth.
During the 4e era, I'd bet good money that more people were playing 3.5 D&D over Pathfinder or 4e.
These days I think third party publishing is also a good metric. Dragonbane is a real sleeper hit, IMO. There's a lot of content being made for it. It's easy to overlook how easy it is to fire up a campaign for a game that has a bunch of PDF adventures at low prices.