@Meddler/Riot: How toxic does a particular theme/identity have to be to warrant a change?

For example if we took Poppy (old vs new ultimate particularly), where her old ult was totally busted, but asides from wall slamming was her iconic "thing".

Or maybe for things like Blitzcrank's hook, Ryze's point click snare/machine gun mage/mana mage (nb: Ryze, LUL), Rengar's TotH.

These are all things that are iconic, and borderline (if not completely) defining for these champions, but either have glaring balance issues, or consume far too much of the champion's power budget. Heck sometimes even both.

At which stage does it reach a: "Time for something new" level?

I feel like this is becoming more prevalent with more and more reworks (large and small scale) happening, and we may reach situations for champions that have already been reworked once, where it's simply not enough, but the problematic pieces that remain are the ones most iconic.

It's not to say this will happen with any and every champion, but it is something I've wondered about myself for quite a while now.