Depends on one's motivation. I really doubt at any point the Dwarf Fortress guy expected to become a millionaire with his game anymore than Notch expected to become a billionaire off Minecraft. They pursued the ideas because they were doing something that they themselves wanted, and enjoy(ed) making.
There are other examples like UnReal World [1]. It's a game about surviving in the wilderness in Finland made by a guy living out in the wilderness in Finland. It had its first release 31 years ago, and the dev is still going at it. He's not exactly rolling the dough, but what's better than doing what you enjoy and making enough to get by doing it?
Well sure, making way more than you need! But spending the prime of your life doing something you love with the chance of a nice payday, seems more pleasant than spending the prime of your life doing something you dislike but with a more guaranteed upper mid payday.
There are other examples like UnReal World [1]. It's a game about surviving in the wilderness in Finland made by a guy living out in the wilderness in Finland. It had its first release 31 years ago, and the dev is still going at it. He's not exactly rolling the dough, but what's better than doing what you enjoy and making enough to get by doing it?
Well sure, making way more than you need! But spending the prime of your life doing something you love with the chance of a nice payday, seems more pleasant than spending the prime of your life doing something you dislike but with a more guaranteed upper mid payday.
[1] - https://store.steampowered.com/app/351700/UnReal_World/