I agree that there are a lot of different factors that may be considered here. And of course, there are different kinds of success.
Sometimes, it’ll be how widespread the game becomes… how much material is produced by the publishers and/or third parties. A good example here is Mothership. They builtup a solid following by releasing the basic rules for free and publishing some modules for sale, then channeling all that into a massively successful Kickstarter, and also allowing third parties to produce material for it as well. Huge success financially, bit also a game with a lot of influence and an active community.
We can look also at design influence. I think one of the big ones here is Apocalypse World. Hugely influential to designers, leading to dozens, if not hundreds, of variants. It spawned an entire branch of RPGs, and inspired others.
Longevity can also be an indicator. Look at Call of Cthulhu. A perennial game, currently on its 7th edition. It continues to sell and continues to hold a significant spot in the industry. Supplements and adventure scenarios continue to be published. It’s also a game that has spawned its own variants… even if it’s more the source material that’s being copied rather than the mechanics. Trail of Cthulhu and Achtung Cthulhu and all manner of other versions are published, and all have some potential appeal to anyone who plays one version. Just tons of material for the setting, and CoC is the one that started it all.
By contrast, you can also have a game that proves successful even without supplements. Blades in the Darkhas been mentioned, and it’s a great example. Yes, it’s also inspired its own design ethos and led to Forged in the Dark games, but so far, no actual supplements have been produced for Blades itself. A couple of playtests and scenarios have been made, but they’ve all been given away fro free. Andthe game continues to sell well (from what we can see), is mentioned often in discussion, is a popular topic of online videos, and has even been optioned for a TV series. All just based on one core book.
I’ll stop there, but I could probably continue with other examples that are unique in some way. Lots of paths to success, I’d say.