A treadmill is when you put in effort but effectively go nowhere.
By this reasoning, it's also a treadmill if a person lives with their parents when they are young, moves into an apartment later, and finally buys a house to live in. You're just living somewhere, right? However, that's clearly an absurd stance to take. There's obviously quite a significant difference between living with your parents and owning your own home. By your logic, how would one get off the treadmill? Become homeless? I hardly see how that's desirable.
Similarly, it seems the only way to break what you claim is a treadmill in the gameplay loop would be to fundamentally change how the game plays at higher levels. For example, in older editions the focus changed from adventuring to domain management. However, some people didn't enjoy the domain management, and therefore largely ignored it in lieu of continuing adventure. I daresay that this was the preference for a sufficient proportion of the player base that domain management was largely removed in later editions, in favor of adventuring.
I disagree that being a wet-behind-the-ears adventurer versus a world's foremost adventurer is just a treadmill. Sure, there are aspects of a treadmill in what they both do there. Fight monsters, loot, repeat.
However, if you pay attention and compare high level play versus low level play, the details show a stark contrast. High level characters have capabilities that their lower level counterparts lack, and can get up to shenanigans that their low level counterparts couldn't hope to. It doesn't matter that their enemies also have powerful capabilities.
I think trying to reduce the journey from Joe Average to Best In The World to a treadmill is quite silly. Joe Average can summon an Unseen Servant while World's Best can wish a clone of themselves (complete with their spell load out) into being. If you want to call that a treadmill, it's a free country, but I couldn't disagree more. Personally, I think you're most likely conflating the idea of a gameplay loop with that of a treadmill. A gameplay loop is similar to a treadmill in that it is a loop, but differs in that it allows for progression.