Steam?
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I'm not trying to compare but even basic flash games of our childhood are selling on Steam for money. They are absolutely weird to sell but for OSM, it would be pretty reasonable to put that on Steam. The Steam community likes free-to-play online strategy games and there is no similar game to OSM, especially that successful (don't tell me about sm20xx). With it, the game can be discovered by players and remembered by the old community. I can't promise that it will be as I imagined, but what you lose?