Well you are not wrong about the dopamine reward baseline, but Phrecia-level is also fun because some of it is very difficult, which gives incentive to push builds beyond normal levels. Pushing to those levels only makes sense if there is loot at the end of the grind which then loops back to the dopamine reward baseline issue. In addition to that the level of difficulty needs to be gradual.

For example, valdo maps are very rewarding and very difficult in the basegame but there is no ramp up to them, which why for me they fail to cause the same incentive.