There are two things here that I don't understand.it's true, in every edition of D&D, gaining better abilities - partly through magical gear - enables the PCs to take on progressively tougher opponents they never could have faced before and that can be viewed as a story trajectory for the PCs.
But I don't think that exempts the game from being a treadmill if it is designed that way. As I pointed out before, it's partly a semantic issue but I think it's an important one for forming impressions. When the assumptions are built in that you need +x items to be able to face your own level of opponents, then I do believe you're looking at a treadmill. You must keep running to stand still.
First, who cares what level of opponents you are facing? Level of opponents is a metagame notion that guides the GM in encounter design. I think this is [MENTION=3424]FireLance[/MENTION]'s point - if my bonuses are +X, then I can safely confront foes with AC of Y. What does it matter what level those opponents are described as?
Second, in what way are the players (or the PCs?) standing still? When my game started, the PCs struggled to beat off twenty lightly-armoured goblins. In a recent session, the paladin on his own held of a phalanx of over twenty well-armoured hobgoblins, driving them back with the Strength of Ten. This strikes me as progress by any measure.
If by "standing still" you mean "still playing a game in which mechanical success depends upon playing with skill, because the numbers on the PC sheet do not guarantee overwhelming success" then that would be true, but to me all that means is that (i) the game is still D&D, and (ii) it is not a Monty-Haul style challenge-free game. In this particular respect, the game is no different from the B/X and AD&D games I GMed nearly 30 years ago - in those games, also, I as GM took steps to ensure that the game remained challenging (and as [MENTION=82106]AbdulAlhazred[/MENTION] pointed out upthread, in doing this I was following advice found in the rulebooks). Although it is true that in 4e, the mechanical transparency makes it easier to set the level of challenges where I want them to be (in AD&D I used Don Turnbull's Monstermark for this job, but Monstermark is both a bit more approximate, and not as transparent, as 4e's monster stats).
As I've just said, I don't follow. You seem to be saying it's a treadmill because the PCs don't come to numerically dominate the ingame environment with which they are engaged (of course they dominate the ingame environment more generally - the PCs in my game started as relative non-entities, and now dominate the politics and rulership of the city where they are based).I realize you don't see it that way but even as you describe it seems to very much support my assertions.
I don't see that as a treadmill. I see that as playing the game. Ever since I started playing fantasy RPGs 30 years ago, it has been taken for granted by me and those I play with that higher level PCs, with their bigger numbers, will face higher level challenges, which also have bigger numbers. Of course I've heard of groups who use their high level PCs to go on inane murder-and-looting sprees in villages and towns, but I've always regarded this as a more-or-less pointless form of the game - you may as well just sit around and free narrate your murder spree, given that the mechanics are playing no meanignful role in the action resolution when 10th level PCs slaughter endless numbers of 0-level NPCs.
I'm also aware that there are sandbox games in which the players, rather than the GM, have principal responsibility for framing the situations. I've got nothing against such games, but I don't think they're the only viable or functional form of play.