Think about it.
So we get these keystones and you're thinking "Wow, that synergizes well with a champ I like to play" right? So you take it on the champ and it feels great right?
Well, no. Turns out that the keystone synergizes ***too well*** with the champion and it makes them significantly stronger. Now the champ is too strong for a patch before being nerfed back down to their original power level.
But there's more. Maybe your champion synergizes just OK with a keystone, while a few other champs can straight up abuse the thing. Now the keystone gets nerfed ***for everyone*** because of a few abuse cases.
So what did we gain from the keystone? Nothing really. We just got pidgeonholed into one viable mastery setup and didn't gain anything from it.
So what is the point of keystones if they're so strong that champs have to be balanced around them? They seem like just another big factor hindering the ability to balance the game properly honesty