Not really sure I agree with most of this. I hit 55 last night and started doing Nightmare sigils, including ones that were +20 above my level. They were not difficult. They were not challenging. There was nothing innovative here. As an example to contrast:
In Diablo IV I get a nightmare dungeon where there are a limited amount of affixes that may be challenging, which I can just skip since so many sigil's drop after your first one. I go around clearing the rooms with limited consideration to anything since most of the stuff is not challenging, the exception being mobs that explode or poison which can be avoided by simply clearing slower and making sure I have a dash available. Some of the stuff requires me to backtrack and make sure I don't miss a single mob in a small room or I waste a few minutes backtracking. I compare this to Lost Ark, where I do a chaos dungeon and mobs auto aggro to me, my pet picks up all the loot and dismantles shit I don't want automatically, and I essentially go from 'easy grinding that's filled with tedium' to 'easy grinding that feels good.' Then we compare boss encounters, which is the 'difficult' content. The bosses encountered in nightmare are copy & paste with no challenge whatsoever. Meanwhile, in Lost Ark, they're insanely complex and challenging. Diablo should have been able to find a middle ground here from a difficulty standpoint; they did not. What they did was create more annoyance and tedium, which is a recipe for disaster to the long term health of the game.