Why dont we just balance defense around pen not existing
Think about it
penetration and the bypass of defense has been the biggest cause of the arms race between offense and defense since the dawn of time, and is why both directions have so many ridiculous workarounds like conqueror, damage reduction, Steraks, shield spamming etc
Why dont we just remove or heavily reduce penetration through items and then balance resists around a point that is lower than now but still strong enough that dedicating to the stat(s) still has a noticeable effect. Keeping it on champions lets you empower draft options, improving roster variety and considerations on a game to game basis.
This way resists are properly strong early game as they're supposed to be but eventually get outscaled as more champions acquire better DPS, while still feeling GOOD to buy these stats, and other options like % reduction and shielding can be reduced. Other defensive choices don't have to be absolutely busted like Steraks shield is because resists on fighters mean something again. and so on