Modern systems tend to emulate Skinner boxes where players are rewarded for simple repetition of mundane tasks rather than being let out of the box and finding their own rewards through setting personal goals and striving for personal PC achievement. Let's get off the treadmill and run our own miles with 5E, and thus find much more satisfaction even if it isn't chucked in front of us no matter which direction or how hard we run.
I think this is a slightly unfair characterisation of at least some of the games being played using modern systems (eg 4e).
The difference being that a system that doesn't have the treadmill built in can still be played in that manner while one that requires the treadmill is the one that forces a playstyle on the group.
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Furthermore, you might find that a system without the treadmill, which many players consciously or otherwise find tiresome enough that it causes such campaigns to "usually end" might be engendered with a bit more longevity if rather than introducing a series of formulaic, set rewards, they require more effort to achieve a deeper immersion and satisfaction stemming from player driven goals. Those systematic, mechanical rewards are really just a veneer to keep players feeling as if they are satisfied in the moment, and don't really hook players for the longterm anyway.
I think you may be misunderstanding the nature of magic item rewards in a system like 4e, in which items of a certain "plus" are treated as default elements in PC build.
It is not a "treadmill" in which the players pointlessly run to stay in place while being tricked (by what means?) into thinking they are really earning rewards.
The pleasure that players receive in watching their numbers go up in 4e is certainly not a pleasure in greater real-world power. Which would, I think, be absurd.
Nor is it a pleasure in a greater likelihood of succeeding at the challenges the game throws up, because a GM who follows the published guidelines will respond to bigger PC numbers by boosting the numbers of the NPCs and monsters. In 4e, to the extent that players increase their likelihoods of succeeding at these challenges, it is because they improve their play, not because their numbers get bigger.
The pleasure in numbers getting bigger derives from the the
ingame implications, namely, that the PC will now be confronting challenges carrying more ingame signficance, and thereby having the potential to carry more real-world story weight. To put it at its crudest, every +1 to hit or damage takes the PC that much closer to being able to confront Demogorgon and win. That is not a treadmill, and not an illusion - it is a genuine feature of the game. Changes in the numbers mean a change in the story - just as the PHB and DMG explain, the fictional scope and fictional stakes grow together with the PCs' numbers.
This is not the only way to play a fantasy RPG, and moreso not the only way to play D&D. It is obviously different to at least some extent from classic dungeon-exploration D&D, because it makes the developing story more important as a reward, and items per se become comparatively less important. But it is hardly a novel way to play D&D - Dragonlance is one early but not necessarily the earliest example (I'm sure some people played Against the Giants and its sequels, for example, more in this spirit than in a dungeon exploration and looting spirit). And presumably at least some of Paizo's Adventure Paths are intended to provide this sort of experience.
Not only is it not novel, but (in my view at least) it is not degenerate or dysfunctional. Nor does it have any particular connection to railroading - AP-style railroads are not the only way to run a game in which story, rather than prospects of success against the challenges the game throws up, does the bulk of the work of providing pleasure to the participants (I'm putting to one side here the generic pleasure the participants might get from playing a game well).
While not everyone wants to play this way, I don't think a game hoping to unify D&D players (in part by stealing market share back from Paizo) can afford to just ignore it, or to assume that it is something that can be tacked onto exploration and looting-style play as an afterthought.
