Yeah, I think this is a big point. As we sort of touched on in the conversation that spawned the "solo AC-min/maxed fighter vs. an army" scenario, due to Bounded Accuracy the relatively rapid level acceleration just doesn't feel that out of balance to me. High level characters are powerful, but they are far from gods. And there's no specific reason every high HD NPC is going to 1) have 100% of the class abilities that a PC gets, and more importantly 2) have the same amount of magic items, optimal feats, optimal spell choices, etc.
For example: A 20th level NPC fighter with no magic gear who wields a brace of rapiers is intimidating as hell, and could tear up a couple of people in a duel. Certainly. But he's far from a universal problem-solver or PC-invalidator. Or, let's look at a 20th level magic-item-less sorcerer whose high level spell list is, let's say.... L6: True Seeing, Move Earth; L7: Plane Shift, Reverse Gravity; L8: Earthquake; L9: Time Stop. He's got plenty of power (and a fun earth/physics theme!), but again, he is hardly a show-stopper powerhouse that can solve every problem himself.
It's all in how you build the NPCs, the world, and the encounters.
I definitely find that 5e mechanics supports my worldbuilding in my Wilderlands campaign, whereas Pathfinder really fights it, and 4e says "don't think about it". High level 5e characters are
powerful, but don't leave me wondering "why does this city bother with an army when the
local wizard/priest/druid is vastly more powerful than all of them put together?" The mechanics seem much closer to the fiction than I have seen before, certainly in 1e/2e/3e/4e.