I remember the "onboarding experience" essentially being losing over and over again and learning from my mistakes, getting a little further or trying a new approach altogether on each new attempt. Maybe it's an acquired taste but by the time I started playing Dwarf Fortress, I was not only already used to that, but thoroughly enjoying it as a core mechanic of games like Rogue, NetHack and Ancient Domains of Mystery.

I don't believe in the concept of "fun in general" in the sense you imply here. I've played very successful games that have evidently achieved popular appeal but that nonetheless turn me off because they're the absolute opposite of Dwarf Fortress: carefully manufactured designs of intentional experiences that you are guided through at every step. Some people really like that, some will want to be thrown in the wilderness with no clue, some want something in-between. Different strokes for different folks.