This is a very difficult thing to ever measure.
You can't look at cons because really only super-fans and industry insiders go to those.
You can't look at 'D&D Beyond' because that's only D&D.
You can't look at Roll20 because there are very large games who's playerbase almost universally reject it (Pathfinder). You can't look at competitors if they're mostly self-hosted.
You can't post a poll because most people won't see it.
All we have is guesses.
So people fall back to sales because if a game is selling well enough to not have to shutter its doors and only sell through the 'discount bin' of DriveThruRPG PDFs then it probably has players playing it "somewhere". Or maybe everyone playing it just already bought the books (somehow I suspect there are more than 0 AD&D1E players out there, yet no one's bought any new books for it in decades).
I also suspect there's a lot of sales through kickstarters that have gone to people who never ended up playing the game they bought. That's just fine for the sellers of a one-off, but it's not generating active community.