Riot won't nerf "problematic abilities" on champions in order to retain their identity.
Instead opting to nerf things around that champion's uniqueness. Examples include Caitlyn traps Twitch Q/R Kog'Maw W Fizz E ETC.
Why? It's to preserve the identity of a champion and ensure that they have a specific niche. When you pick these kinds of champions, you tend to do so because a certain part of their kit sticks out above the rest. It defines that champion, makes them better in certain scenarios, but also (in theory) allows for a trade off because they should be lacking in other areas.
They've said this before, stating (I apologize for not directly quoting riot) that they want to preserve "iconic champions" or "iconic parts" of champions.
This is a good design choice IMO because it keeps champions from feeling samey and provides a specific reason to pick a champion. The issue with this, and I'm going to put it in bold in hopes that I'll catch the eye of people who don't actually read my whole post before commenting.
#This doesn't always work. Sometimes a champion's abilities need to be changed because they just suck.
Examples include Ryze W Pre-rework Poppy R Basically everything about pre-rework Yorick ETC.
Sometimes things absolutely deserve to be changed, but the point I am trying to get across is this. #Asking for nerfs to a champion's main ability such as Kog'Maw's W is rarely going to go through unless the ability is way out of line. Riot will not nerf what makes a champion niche because it will remove them from their niche. If they were to do this, we would slowly slide back to how champion classes were in season 5 where ADCs were fairly interchangeable aside from stat differences and minor differences in mechanics.
That is extremely different than other ADCs who build like this
Xayah: 
Vayne: 
Trist: 
for their ability to kill tanks?
There is still variety within the ADC class as to who is and isn't the best at doing a specific task.
