You (almost) never want to nerf a champions identity.
Unless their identity is fundamentally broken. (E.G. ekko) Nerfing a champions identity, in this case let's say ashe. Nerfing ashe's CC is a stupid idea (unless it is ridiculously overtuned). CC and kiting is what makes ashe unique. If riot were to gut her passive but let her keep her damage, then she would be "just an ADC". There would be nothing unique about her. Instead what riot tries to do is nerf parts of their kit in compensation.
Another example is jinx. You pick jinx for 2 main reasons
-her passive
-her Q
Let's say people are sick of her passive so riot reduces it to just a tiny MS and AS buff, now she's got nothing going for her but her Q, something that can be made up by just picking 
for the range and extra damage with headshots.
If they nerf her Q then she is just an ADC with a steroid on her passive.
An example outside of the ADC role.
's ult is too powerful.
He can engage too hard and live too long.
Riot guts his ult CD and puts it at a 3 minute cd at rank 1 2.5 at rank 2 and 2 at rank 3. Now there is little to no reason to pick malphite anymore because his engage (the main part of his kit) is useless.
(there is probably a niche analogy in here somewhere)
