One might as well say that AC has to be either wrecking the fighter or totally irrelevant.
AC is pretty much irrelevant in a game where standard attacks are the only ones used. It only really becomes a factor when a method of lowering attack rolls is used (ie power attack), or when you use a full attack (increasingly lower attacks). If you make the AC high enough to not be irrelevant in these cases, then you are wrecking the fighter by making all his attacks miss.

AC is actually a really bad example of a working similar system if you want to argue SR isn't working. When you consider SR is rarer, supposed to be powerful and meaningful at all times, it being a random pass/fail mechanic is very meh.


Spell res isn't fire res, it's like a neutralization field, it's magical.
It's a neutralization field that can just be ignored 99% of the time when dealing with equivalent CR encounters. Hardly neutralizing.

You might as well try and rewrite AC into a reflex save as the leather armor would block damage.
Armor as DR and avoidance mechanics being separate is a whole 'nother box of worms that will probably be opened down the line. This is for an alternate spell mechanic.