The good thing about armor class is that even if it's a dragon, you can still wound it even at low levels. So, a ton of grunts can still kill a dragon.
Meanwhile, DR can make some enemies unkillable.
Imagine a dragon with DR. Yeah. That would be no fun.
Why is it good to be able to wound high level creatures at low levels? What's wrong with e.g., an Ancient Dragon taking no damage against 500 commoners (or 1st level fighters) with bows?
Lol. Imagine Smaug in D&D 5e. How you gonna kill him with a single arrow?
One system I liked was Star Wars RPG. Wait. Which version. Might have been WotC. I'd have to pull the book out again. Anyway, they did Vitality and Wounds. Vitality could get up to like 100 or more points. Vader had something like 120 Vitality. Wounds only got to maybe 15 at the most. Normal hits lowered vitality. Critical Hits lowered Wounds. You could literally get a critical hit and kill an enemy with 1 shot.
Problem with that was, players killed Vader with 1 hit one time. Lucky dice rolls, and the Dark Lord died. No armor could save him.
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