Doom and Quake are still making money. You can open-source the engine but charge for the art and levels

Sure, provide the game for free but charge sats for something inside the game from your storefront in there. Skins/weapons/whatever: Simply become a merchant.

https://nikke-en.com/opensource/

yes nikke does that.

It would certainly be possible for the community to sustain it and keep it running.