AC is more than a dex save… And in fact may not involve dex at all if the target is wearing heavy armor.

I'm well aware. I just said that Dex is represented twice, once in each "system".

There’s nothing arbitrary about it

I mean the division itself is entirely arbitrary.

a unified defense system would no doubt involve a lot of special-casing/ad-hoc calculations to be at all worthwhile, to the point at which it would be far more cumbersome and confusing.

What? No? Why would that be the case?

The two systems could be mostly merged by just having saves be passive, and having the attacker roll to overcome them. Exactly like AC currently works now.

Let’s say we only use AC as a defense and have no saves. How does a spell like Hold Person work?

The caster rolls their spell attack against the target's Wisdom defence/save. Exactly like how AC works now for physical attacks.

Buffs like could add a flat value to the defence stat, like AC buffs currently do.

The more I think about it, this would actually streamline combat a bit too. Because the "acting" party is doing all the rolling, rather than waiting for the defending party to roll a save to see if attacker gets to continue.