Like running 5e without making rulings, it also wasn't how 4e was designed to be used.
The game made it clear what could challenge a party at various levels. If the DM wanted a door to be an obstacle to an 18th level party, he placed an iron door, if he didn't, it might have been wood - if he wanted it to be impassable, adamantine. That was no different than it ever was, or is now. In 5e, if the DM wants an impassable door, he narrates it as such, for instance, because bounded accuracy requires narrating success/failure when you want it - the system virtually always leaves some chance of success at an impossible task or failure at a trivial one.
4e just gave clearer guidelines and scaled dramatically enough that trivial was trivial and impassable, impassable, without resorting to fiat (or, really, while presenting an illusion that setting a DC wasn't, in itself, DM fiat/judgement/rulings-not-rules).
The point you're trying to get at is that 4e had dramatic, consistent, and even somewhat balanced, scaling with level. 3e, in contrast, had very dramatic, imbalanced & inconsistent scaling, while 5e has somewhat consistent, quite muted ('bounded') scaling (of d20 bonuses and DCs - hps/damage scale quite dramatically). And, of course, the classic game scaled dramatically and very inconsistently, with a quixotic attempt at balance over the long run.
Among the various editions, 4e & 5e are actually the most similar in this regard, because they are the two
most consistent in their scaling. If you were to run 4e very close to the guidelines in a 'scaling world' kind of way, you'd get a result very similar to that delivered by 5e bounded accuracy if one were to simply roll all the time. Similarly, if you exercise DM judgement 'well' in deciding when to narrate successs/failure and when to roll in 5e, you could get a result very similar to that of 4e's dramatic advancement/relative-balance. Ultimately, both editions keep everyone more or less 'on the d20' (that is, a roll that one PC can fail on a 1, the 'worst' PC at the same thing can generally make without needing a natural 20) when checks matter. 5e, unless you exercise DM judgement to narrate success/failure (
which, dammit, you're meant to do), also does that for everyone/thing (a roll a 20th level character might fail on a 1, a Kobold might get lucky and make without needing a natural 20), shattering any illusion of advancement. While, contrarily, 4e, unless you willfully level the world with the PCs (
which, dammit, you're NOT meant to do), does not.
Prettymuch 'bounded accuracy,' right there.
Also similar to 'sweet spot' (~3-8, IMHO) play in AD&D. Relatively little advancement, thus avoiding shifts in tone, and (again, IMHO) more importantly, the radical imbalances outside that range.
Your complaint is invalid - or, more accurately, your complaint is based on what you felt based on a misperception. You perceived the tools that gave the DM the option of presenting neatly-balanced challenges to PCs at any level, as defining a world in which everyone was presented only with neatly-balanced challenges, at every level. 4e did not define a world, at all, and the same tools that allow a DM to craft a neatly-balanced challenge allow him to create a very challenging, unwinnable, or trivial one, as well.
The very concept that a guideline might actually work can be hard for a long-time D&Der to reconcile himself to, lying so far outside decades of practical experience.
That follows from your preference for E6 - 3e's version of bounded accuracy. If I may be judgmental for a moment (and, I think with your willful provocation in re-hashing edition war misinformation, you've earned it), I would have to say that it's clear your preference is derived from the failings of earlier editions, in which high-level play tended to be problematic and coping meant sticking to the sweet spot (mid levels in 1e or E6 as the most robust case of 3e), leaving you unwilling to accept an edition that resolved those issues.
Well, I mean, or you're just trolling like a champ. In which case, a round of applause, and may you never take fire or acid damage.