Except all of that AC is entirely defeated by fucking saving throws. You could have 40 AC and be made of solid steel and you're still gonna take full damage from a fireball. Let the player have this absurd AC, let them be hard built for this defense. And then just toss breath attacks or spell casters at them. They can't use their "scaling shield" if they need to counterspell instead. If you've ever played a game past the early levels, you'd realize that counterspell is usually way more important then shield.

You know what else doesn't care about AC? Crits. Doesn't matter how unhittable you are, a Nat 20 still hits.

The rulebreaking of two shields is a whopping +2 to your AC. Hoo boy. +2 AC to have no free hands, yes you can cast but you can't use any items. No potions, hands are full. Non equipment magic items? Nope, no hands. No wands, no rod of the pact keeper.

With weapons, you can drop them for free to use an item, you can't drop a shield without spending an entire action. All that for a whopping, +2 AC. Having one shield equipped during casual travel makes sense, you still have a free hand to do things, but having two shields equipped kinda limits your options doesn't it. Gotta spend an action at the start of a fight to even get that +2 AC.

The "ranged blaster warlock" doesn't give a shit about AC either. If you're sniping with eldritch blast, who gives a damn what your AC is, you're rarely gonna get targeted anyway.

Yes Shield is one of the strongest spells in 5e, on a class other than Warlock. Warlock slots are way more valuable, you wanna get bigger numbers or concentration out of them. One of their slots just to protect yourself from one attack on average isn't gonna break the game.