Back in February, Valve released the source code for Team Fortress 2 (TF2) into the Source SDK, which allows modders to see how it all works and get their standalone mods onto Steam. But also, it allows them to fix up issues directly in Team Fortress 2.
It does indirectly support the cosmetic market, from which Valve still makes money I think. It’s kind of unfortunate that people work for free to support their income, but it’s not worse than the game dying entirely, probably.