I totally agree about level scaling, HOWAYVER, as a D&D DM I've seen firsthand what happens when the players end up in a low-level area you expected them to tackle earlier.
The mountain valley has a ruined city in the middle, where four small villages have sprung up under four mid-level friendly NPCs. There are also four main dungeons around the valley which all connect. The boss dude at the bottom is trying to get four antagonist NPCs to bring him all the pieces of the MacGuffin so he can slip completely into this world - or they could banish him completely. Either way, you need all the pieces, and these four antagonist NPCs don't want to share whatever rewards there are from the boss, so they all struggle against each other.
There are also dozens of small dungeons scattered around and more than four MacGuffin pieces.
The problem is I wrote up the dungeons when the game started, so each of the four could be tackled at roughly the same party level. Let's say one of them was easier, and its four dungeon levels before the conjoin with the others at the boss were at difficulty 1/2/3/4. Two of them were harder, more like 2/3/4/5. One was the hardest, more like 3/4/5/6.
The structure of the dungeon allowed for people to plumb to the bottom of one dungeon, recognize they're connected because the boss says "hey man, posh rewards going out to whoever brings all the MacGuffin parts", and go through the bottom entrance to the next one. That's the hardest part of the next dungeon because the antagonist NPC there set up defenses to keep the others from ambushing him. But if you get through, it's a shortcut to the higher-difficulty segments.
My players cleaned out one dungeon, noted that the entrances were all tough to get into, and left to go into another dungeon from the top. Becoming a little bored with how easy it was for their 5th-7th level characters.
Here's my hypothetical solution to dungeon scaling vs. static dungeons that are boring because you go in at a super high level.
One dungeon has, at the end, a strange ritual that you can interrupt and break the thingy. If you break it, a bunch of evil spirits get released and scatter throughout several dungeons. If you let the ritual finish, you fight a big boss guy and a bunch of his cultists escape, setting up shop in several dungeons where it would be appropriate. Either way, you finishing the dungeon makes a bunch of other dungeons harder but also more rewarding - the cultists gather loot, and the spirits drop ectoplasm or whatever.
A sewer level has a valve at the end and if you close it, the outflow is no longer tainted so the waters around the town no longer spawn mutants, but the deeper levels of the sewer become backlogged with the nasty stuff and the monsters inside are tougher. Or what if you open the valve, weakening the monsters deeper in but polluting the nearby waters completely? Maybe there's a holding tank for corrosive stuff that you can release into the dungeon to open certain pathways and change how the sewage is distributed.
Help a wizard in the guild defeat his rival, and the rival is no longer a concern - but the wizard you helped is now doing some stuff.
Seven wizard-princes share the power of their father the conjurer-king. If you slay a prince, his powers are redistributed to the other princes making them more powerful.
If you defeat a gang of bandits, some will inevitably survive. These will flee to other bandit gangs, slightly strengthening them. Without the bandits' depredation of the roads, the local Thieve's Guild (which controls the teamsters) flourishes. The bandit camp, over time, becomes a lair for orcs or something, and with this forward base they're able to strike deeper into inhabited lands.
Anyway, the general theme is that completion of a dungeon (which is kind of like passing a test showing that you have a certain level of character development or player skill) causes specific changes in the world, which include changes in difficulty in various areas.
EDIT: Way too many "of course"s