Your point is good. Just exploring thoughts, I really don't have a counter point
Another fun thought because I love them, in the world of stupidly complex mechanics, if a class gained opposing resistance temporarily whenever they took damage. And lost damage type resistance whenever they TOOK damage.
Note numbers are not tuned AT ALL but example - Gain physical resistance equal to magic damage taken and vice versa. Whenever you take a hit you lose 15% resistance of the type of damage you took. So if at 50% physical, you get smacked by a zombie you are at 35% resistance(but just gained a bunch of magic resistance).
Mechanic with whatever balancing would not always be ideal based on monster balance but could lead to lots of fun mechanics.