A guy with the same DEX, same armor and same shield has the same AC. Period. If that guy is L1 his AC is X. If that guy is level 17 his AC is STILL X.
Can you do THAT with a 4E style system?
In 4e, his AC isn't still X, because in those 16 levels he's learned to dodge and avoid damage better. Which is odd, because his hit points also increase because . . . he's learned to dodge and avoid damage better. Hrm.
I'd actually prefer a game where HP is static, and defenses increase as you level up. So in a hypothetical 5e, my logic would go like this:
Normal Human
He's a person. Based on how well-trained (or divinely gifted, or magically infused, etc.) he is, we'll be able to set his minimum defenses. Someone who's fought a thousand demons and now is working to kill a demon lord has somehow managed to avoid death. So based on that narrative element, we decide that he's 20th level.
Since he's 20th level, all his defenses are going to be at least 20 (10 + 1/2 his level). If he puts on plate armor, his AC jumps by 8. If he's got a high Constitution, his Fortitude improves. If instead of plate armor he's got a magic robe and a high Intelligence to predict attacks, maybe his AC and Reflex jump by 6.
So say after figuring out what all he's got, we determine that he's a nimble scimitar-wielding dervish in light armor, and has AC 26, Fort 22, Ref 25, Will 20. If he's a PC, the GM might compare his defenses to what's listed as average for a 20th level PC and see whether he's in the right range. If he were a bit underpowered, the GM might figure out a way in-game to strengthen him so combat is better balanced.
On the other hand, if he's an NPC, the GM could go to the trouble of figuring out what equipment the guy has, calculating stats, and then checking to see if it's balanced. But if the NPC is mostly just there for a combat encounter, the GM can decide "I want a scimitar wielding dervish," then as a short-hand look at a table of 'standard defenses for level 20 opponents.' He could pluck stats from there, then maybe pick one of three template modifiers (Heavily Armored, Magically Defended, Agile Foe) and tweak those stats. Both as a PC or an NPC, he'd have, like, 20 hit points.
Now to me, the following two methods are practically identical:
1. The party's 20th level, and I want to throw a bad-ass dervish against them. To make him a challenge, I'm going to give him enough stuff so he's got defenses in the low 20s.
and
2. The party's 20th level, and I want to throw a bad-ass dervish against them. I'll trust this table that says to give him AC 25, Fort 22, Ref 22, Will 22, and to make his stats match his flavor, I'll use this template to tweak that to AC 26, Fort 22, Ref 25, Will 20.
Demon Lord of Brick Walls
You mentioned wanting to make a demon lord who's super tough but with a sucky Will. Great. Now compare how this works in 3e vs 4e.
3e Okay, I want his Will to suck, and his AC to be high. Hm . . . well, if he's a demon lord, he's got to be able to survive what 20th level PCs throw at him, so I guess I need an AC of at least 25, but I'll kick that up to 35 because he's tough as bricks. How do I get that? Well, I could just give him natural armor. That works. Okay, his natural armor bonus is +25. Now I want him to have a lot of hit points, because he's a demon lord, he's tough. So he probably needs 40 hit dice, and a Con bonus of +10. That'll get him 600 hit points-ish. Excellent for the demon lord of brick walls. Okay. Now for his sucky Will. Well, he's got 40 hit dice, so as an outsider that means he's got a +12 Will save. But I want it to suck, so I'll give him a . . . well, a Wisdom of 4 is really low? I mean, who wants a dumb demon lord? But sure, he's just oblivious, dumb as a stack of bricks. Boom, Will save of +8. Excellent.
4e Okay, I want his Will to suck, and his AC to be high. He's a demon lord, which means he's going to be, let's say, level 27. Okay, so as a default he'd be AC 40, Fort 37, Ref 37, Will 37. But eh, let's switch that to AC 44, Fort 41, Ref 37, Will 30. Or y'know what, screw that. Will 10. Done.
You've accomplished the same thing, but the second option is a lot faster, because you don't have to look for ways to justify your choice. You just make a choice that works.
Demon Lord and His Twin Brother
If the demon lord's skin isn't tough as starmetal, he probably shouldn't have survived very long as a demon lord. Someone would have stabbed him. So he probably already has starmetal armor, or some other item-based way to improve his defenses.
You, as the GM, have invented this guy, and are responsible for deciding that he is tough enough to be a demon lord. Moreover, you're deciding what 'tough enough to be a demon lord' means. In the real world, the toughest mofo might only be equivalent to what a 5th level fighter in D&D is. You could certainly create a game system where a demon lord is a level 5 foe. But you, as GM, decide that a demon lord is tough enough for a 20th level party.
To match the mechanics to the narrative, you decide to give him the best resources available. He's got a cloak of resistance and robes of the archmagi and bracers of armor. And after all that, you decide "Huh, this guy's AC is still pretty low compared to other demons. I mean, if this weren't a game I wouldn't have to worry about it, but because it is a game, I want to justify why he hasn't been stabbed yet and replaced. Well, other demons have natural armor. I could give him natural armor."
And just there, you made an arbitrary mechanical decision, based on a gamist desire to make him appropriately tough. Some of the 3e monsters have just stupid natural armor bonuses, like +20, with no explanation where that comes from. It's just there to make them have the right AC for their level.
That's why in 3e it was so much easier to make monsters as high level foes, as opposed to NPCs. NPCs made GMs think they had to use PC rules, but PCs have to jump through all sorts of hoops to get high enough AC. Stats, armor, feats, magic items, spells. And players like jumping through those hoops, because they like having their preparation rewarded.
But monsters are made to fill a role. The GM needs guidelines on what's fair for his party, and then he can go back and forth between narrative and guidelines to create something compelling. But if you try to build an NPC the same way you would a PC, a) it takes too damn long, and b) you as the GM ultimately have to decide to set a power level anyway. So why not just accept that 'power level setting' should be the first step?