What is the point of a keystone/item being very good on a champ?

I mean think about it. If a new item or keystone comes around that synergizes really well with a champ, either the item, keystone, or champ is going to be nerfed down to a level they were before the change. All it really does is enforce a cycle where one viable keystone or item is mandatory on a champ.

For example, lets look at Irelia. Irelia is practically balanced around how well the current iteration of is on her. It's pretty much a required item because she is balanced around having it. So what is the point then? It's not really a 'good' item as much as a 'required' item at that point.