Ehhhhhh, this is your analysis from a specific perspective. That's OK, but I would say:Exactly, and IMO this steep power curve (or narrow challenge band, to use your term) is a very serious - almost fatal - flaw in 3e and 4e design.
I mean, a game where Merry and Eowyn can't punch above their weight to bring down a Ringwraith isn't a game worth playing.
1) 3e and 4e are totally different games, with vastly different design goals. As I see it the problem 3.x/PF/etc has is that it wasn't designed with enough focus on GAME. A HUGE mistake was made, basically. 3e's job was to rewrite 2e and turn AD&D pea soup muck 'rules' into something that was extensible and realistically playable as-written. Most of the approach seems to have been to try to create a more 'realistic' core, or at least a more 'procedural' one in which most of what would happen in the action at the table could be referred in a general sense to the rules and some mechanic applied to it. I guess they simply didn't get as far as "does this mechanic actually work to produce a playable game?" From what I've heard they basically playtested in a small closed group who's approach was to take 2e characters and material and translate them to 3e and see how it played. Apparently nobody thought to then start a 3e campaign and see how it would evolve, because if they had, and had they done so with their eyes open, they would have run into a lot of bad problems. The result of all the bad problems is, 3e really is NOT extensible, because it is broke at the core. 3.5 attempted to fix it, but it was way too little.
So, maybe it is fair to say that 3e has a 'narrow challenge range', though I would go with the more severe "Except below 7th level or so, challenges DON'T WORK AT ALL in 3e."
2) 4e, with its total focus on game and playability, IMHO cannot really be tarred with this criticism. The design FUNDAMENTALLY presupposes that challenges are dramatic tools and automatically provides for the PCs to triumph as the default, assuming the players want to and actually try. In 4e a Ring Wraith, appearing in the night on Weathertop, would be a level-appropriate creature, part of a level appropriate encounter. I'm not sure what level I would assign to the hobbits at that point, I don't think that is an easy question, but clearly it was a highly difficult (say level + 5) encounter. Maybe even higher, as it was the result of failure in at least one SC and thus made more difficult than normally likely! Honestly, I wouldn't even handle it as combat, given all the factors, but I would just remind you that the Ring Wraiths didn't seem to really have a fixed level of power, even in the original story. They were animated by the will of Sauron, and their abilities waxed and waned as his focus was on them, and as his fortunes rose and fell. All of the Nine together, fully mustered for war and at the focus of their master's attention would be immensely powerful. A single, or a few, Wraiths, operating far from their master's power base and without his principal attention were weak enough that fire and a prayer to Elbereth drove them off temporarily.
I think it is perfectly feasible for 4e to present the above, whereas a game like 3e would find it antithetical to its central design thesis and mechanics. The 4e version would be perhaps a level 6 undead lurker (or just a part of an SC without needing a stat block as such). The same being, might appear at the head of an army as a powerful paragon solo monster, finally being defeated only by the coordinated action of two higher level PCs and a lot of their followers and minions (or again, as an SC).
It is fair to say that 4e handles combat and similar stuff more in an 'action adventure' mode than in a literary "battle of good vs evil" mode. So it isn't the best game to do an LotR kind of scenario, but not because of 'narrowness of challenge rating'. Honestly, how wide is the challenge rating of an orc in AD&D? 5th level PCs will basically laugh at orcs. Yeah, 100 orcs is problematic in mostly a logistical sense, but as actual combatants they're basically worthless. I don't see much of a 'wider challenge range' there. Maybe a tiny bit, but again 4e handles that by having several differently leveled orc stat blocks to play with.