Something that annoys me: champion identity vs feed (LOOK INSIDE)

So this is kind of a pet peeve of mine. The problem is, when a champ is fed, even if they are only a few kills up they become so much more powerful that it doesn't really matter what they r supposed to do - within the limits of their kits, they can do what they want. Let's take an example: Riven. Now, i play riven a fair amount (please don't downvote just for this), and while she may be a little overtuned in soloq imo she is not particularly broken. The problem with her and champs like her is that when she is fed, her whole kit instead of just what she's good at gets insane. Riven is supposed to be a squishy offtank with good dueling and multikill potential. She is NOT supposed to be able to tank through an adc and mid laner with her shield and lifesteal. The same applies to underfeed - IMO champions like vayne should basically be able to duel anyone (ofc, provided their mechanics are good) post 35 mins.
TL;DR: Champion's getting fed increases their power at things that they shouldnt be good at too much, and being underfed destroys their identity too much