This is a terrible idea. You could have a billion magic armour, but because it's only 50% of your max magic armour. You suddenly have the same chance to be charmed as someone with 1/2 magic armour.
The whole point of armour is to shift the focus onto healing and targeting weak armour. This system means you dumb shit on the enemy and set it on fire and hope it works.
If you think rng is somehow the epitome of strategy, you're deeply mistaken. "Always bet on black" isn't something a game aspiring to have strategic combat should aim to focus on. Betting on rng isn't a good strategy it's a sign of gambling addiction.
The system is fine as is, it may need some number tweaks (and more aoe armour restore skills), but having a degrading chance to resist cc would be terrible.
gambling on some rng cc affect is not a deep strategic decision. It's just a sign of gambling addiction.