Why diversity in the bot lane carry role would improve overall balance

This became a bit longer than I expected, TL;DR at the end

So I recently read a post from a Rioter (think it was Meddler) about Gnar. What he said is that Gnar is a very interesting and important champion because of his relationship with juggernauts (let's take Darius as an example): the existence of Gnar means that Darius can never get too OP, but because a lot of Gnar's power comes from his ability to counter a class that isn't picked in every game, he will never be completely broken himself either. He serves as a very strong "class counter", and he alone is enough to prevent a whole class of champions from becoming too broken.

Gnar is a very strong example, but there are many more of such class counters, even between roles (Vayne deals with tanks, Kassadin is an anti-mage, etc). Darius is a great example of a champion that usually (there are some extreme exceptions, like post-rework Darius) won't go completely out of control, because in every role you can counterpick him: Gragas brings disengage from the jungle, Anivia can build a wall to keep him out, Kalista can kite him for days and Janna will make him regret his pick. Though not every class can be counterpicked from all 5 roles, there is usually at least 1 champion in every role who isn't straight up countered by the class. Those class counters are very important to prevent strong champions from becoming OP champions, and there are enough of them to make sure it's rarely the case that a single class crowds out all others (tank meta was a recent exception). In some cases a whole class is a class counter: every enchanter loves to make the life of that diver, who wanted to get on top of your backline, a hell.

There are many more of such class counters (Vayne deals with tanks, Kassadin is an anti-mage, etc). All of them rely on 2 conditions to be healthy:

1. When a certain class gets strong the champion can keep them in check

2. Because that class isn't necessary the champion will never get out of control (except for old Kassadin, but that was something entirely different).

And here comes the thing. The title already gave it away: there is 1 position in the game which is filled by 1 class. In the bot lane carry role there are only marksmen/ADCs, which means there is at least 1 ADC in every game. A champion or class that counters ADCs (let's take assassins, and then Zed as an example) can therefore be extremely broken. If Zed is too strong, he can go in every game, because in every game there is a sweet, squishy ADC for him to prey on. Zed is a class counter, but both conditions don't apply, making the relationship between Zed and marksmen unhealthy. The existence of Zed isn't enough to prevent ADCs from being necessary, and because they are necessary there is no limit for Zed. If Zed (again, Zed is just an example, this goes for any assassin) is too strong, he immediately becomes way too strong.

Let's take a look at an attempt from Riot to create a completely different bot laner: Mordekaiser. It didn't work out, but with some more work it could have worked, so what if it did? If Morde was a viable bot laner, an assassin meta would be a lot less dangerous (because assassins only have a purpose when there are enough squishies to blow up), and so would an ADC meta (because assassins can actually prevent ADCs from being picked). Because there is no alternative for an ADC (squishy ranged damage dealer with little to no self defense) this is not the case: anything that counters ADCs will be way too strong, and ADCs themselves are either way too weak (they are weak but you still need to pick one every game, even in bad situations), or way too strong (they are strong and there is nothing that can stop you from picking one).

I'm not saying that Riot should instantly turn champions into viable bot laners: Morde showed us what happened when you try to make a big change without doing much to his kit. But for the long term, it would be beneficial for the overall game balance, as it would make prevent ADC counters from instantly going from "above average" to "ridiculously OP".

TL;DR Every class has its counters, Gnar to juggernauts for example. Gnar is strong against juggernauts, but because not every game has a juggernaut, Gnar won't instantly become god tier when he gets a little bit too strong (against Gnar you can always pick Yasuo instead of Darius). ADCs however are in every game. This means that when a class or champion that counters ADCs gets a little too strong, you can pick it every game and the enemy can't pick something else to stop it, resulting in the champion or class becoming instantly overpowered. Because of this, it would be healthier to have not just ADCs in the bot lane, but also something like Morde, who can deal with assassins and such stuff.