Yeah largely I think the problem is that the treadmill argument is only meaningful if difficulty becomes this entirely abstract thing-- if all of the things in my dungeon for level 5 characters, and all the things in my level 15 dungeon are level 5 and 15 challenges respectively, and there isn't even flavor difference between them, you're going to see a treadmill. But obviously, there's serious problems with the fiction if that's happening-- If you're immediately replacing Young Dragon statblocks with Adult Dragon statblocks, you're gonna get that sense, but let them fight a bunch of young dragons? they'll feel how much more powerful they are.
The relative power level of challenges should be reflecting their relative difficulty in the fiction of the game world-- you should not be leveling basic zombies or something up with the players, their stationary power level should serve as a measuring stick for the concept of growth. Players feel great when they're decimating hordes of creatures that used to challenge them as solos or small groups, not using the players level to pick DCs can mean that non-maxed skills can be important, particularly if you're running exploration mode correctly and everyone has a set activity they're doing as you move through the environment, so things can't always be covered exclusively by specialists.
If there can be said to be an issue, its that what we call the 'game' is really a rules engine, and therefore a lot continges on the adventure design, a GM who makes all the DCs standard DCs of the parties level is naturally going to create that feeling, while a GM who respects that a simple door is a low dc, or that they can let the PCs encounter monsters weaker than them they've seen before to telegraph their growth, really won't have that problem.