Greg,
I hadn’t heard of NetHack, but after reading a bit about it and other intentionally/ mindbendingly hard games, it made me wonder if there’s another category of challenges in which you face them not in hopes of mastery or victory but to experience the journey of failing again and again (presumably in interesting ways or else the player would walk away). And I agree with your point that it’s not an all or nothing proposition. The game landscape is more robust for having both challenging and challengeless games.
-Josh