The most overlooked reason why champions can not be nerfed currently.
I'd love to say that the most obvious reason champions should not be nerfed is overlooked, but I think it's more accurate to say that it is glossed over instead. The amount of outcry from the devs on how many champions they want nerfed has increased dramatically lately. You've no doubt heard about the main champions that designers want to nerf, but the more involved you get in the community, such as listening to their personal livestreams, or seeing their opinions typed in various chat rooms, then you'll quickly see how deep this goes. The list of champions they say they want nerfed is extensive and includes heroes you may not realize like Hulk, Juggernaut or Scorpion.
There is an extremely clear and objective reason why nerfing any champion should not be considered any time soon. This game, as they have stated on the live streams, is built around intentionally charging you a resource to participate in content. This just means that at a minimum you will be spending something to participate, such as energy, and at maximum(necropolis) you will be spending a very significant amount. This focus on skilled performance is why a lot of us end game players love this game, but unlike other games that just take your time or effort when they are bugged or broken, this game takes your resources, which often leads to taking your real money. The issue is that this game has been critically broken due to inputs, animations, and AI over the last 3 years, but it has continued to charge you for mistakes. Normally this would lead to a game's downfall, but summoners have adapted and learned to use the champion designers' mistakes to balance out input, animation, and AI mistakes. This unintentional balance has acted as life support for a game that they have been unable to repair for 3 years. For example, if parry failed and you were using Hercules then it was still fair. No harm... no foul.
The developers are doing their best not to mention how broken the fundamental aspects of this game are, which seems fair enough not to dwell on them if there isn't a fix in sight. Due to this, however, none of the conversations that they have presented on matters of nerfing our champions take this balance into consideration. By doing so they completely gloss over the objective fact that this game would be extremely unfair fundamentally without champions that help us deal with a poorly functioning game designed to charge you for its mistakes. If you can't fix the game, but still want to nerf the one thing that keeps this game remotely fair, then for the life of this game, you will have to find another way to bring balance to how unfair it would become.