I still think towers, minions, and monsters should do 50/50 physical and magic damage
Right now tanks basically stack armor and slip a single mr item into their build when they absolutely must. Ambient magic damage is so sparse in this game (oh no I need spirit visage or gromp's gonna fuck me up!) armor is almost always going to be more cost effective unless the enemy mage is bursting you out.
The fact that tanks are mostly at their leisure to stack armor means that armor pen needs to be stupidly strong, which means that when a tank really needs armor (against a full ad team, for example) it doesn't hit as hard as it could.
If we added more ambient damage, mr would be more important, and armor items could be allowed to work. A win for everyone involved except the 400 armor Malphite tanking 400 tower shots on live.