Balance or somethin

If a champion heavily relies on one item..like a slow or a certain active, why not give that certain champion what it needs, and then nerf the item so other champions can't abuse the item. Of course, this only happening with items that have a big problem. I'm not talking about giving Lux more CDR and then nerfing Morello's just because it's a core item on her and can be abused by other champions. (Not that it is, just an example.)

?