Keystone Masteries Can Never Be Balanced Without Being Useless

If you have a mastery which is on-par with an ability or an item, it will never be balanced unless it does next to nothing.

There will always be an abuse case.

Let's say Thunderlord's gets nerfed so it does nothing. That's fine until you get to Grasp, which gives you a 100-120 damage nuke+lifesteal on a 4 second CD just for autoing as a tank or bruiser. Let's say that gets nerfed. That's fine except for the obvious abuse case of Strength of Ages for junglers, since it's essentially a free Giant's Belt (1000 gold worth of stats) without needing a slot. So you nerf that so they do nothing.

Then you have Fervor of Battle which is still really strong on cases like Jax or Yi. Oops... you see where I'm going with this? You keep nerfing, everything does nothing, fine - but then what's the difference with the old mastery system if the keystones do nothing?

It was much better when masteries were for plugging in small stat holes in your early game or some minor additional late game scaling or some extra utility; that way, champions were balanced almost solely through their kits alone, and their base stats were variable. Right now it's champion's kits that are essentially variable, and their base stats are NOT variable since almost none of the masteries except 2 help early game.