idk, maybe your definition of successful is different from mine. But imo a sucessful game is one that have a good amount of players that really enjoy your game, and maybe your game even got a community around it. And a sucessful game dev imo is one that can use game development both as a fun hobby and a potential source of income, so that they don't need a boring job along making games (basically being self employed, like a freelancer or smth).