A closer look at the shallow ADC pool: 4 champs with combined 100% play rate
We can keep talking about how shallow the jungle pool is, or how it's only Courage tanks top, but looking at the stats ADC is actually the role that is being dominated by a very small amount of champs.
Look at those stats
: 52.53% win rate and 30.07% play rate
: 50.68% win rate and 24.6% play rate
: 49.97% win rate and 22.77% play rate
: 51.17% win rate 21.08% play rate
Those 4 combine for a 98.52% play rate, which means that on average you will see 1 of these champs in almost every game. We can add the next 2, 
(51.65% win rate + 15.87% play rate) and 
(53.05% win rate (the highest) + 13.15% play rate) and we get over 125% play rate with 6 champs who all have a more than decent win rate, only Ez being very slightly below 50%.
Let's look at the other lanes: how many champs does it take to get a combined 100% play rate and what is the win rate of those champs?
Top: 
combine for 99.43%, that's 13 champs with none being over 52% win rate
Jungle: 
(8 champs) have 99.71% together. Though their win rates have much higher differences (Shaco and Vi over 54%), largely because of the large jungle changes (it will get better over time), it's still twice as many as for the ADC role.
Middle. 
make 99.04%. From those 10 only the first 3 have 50%+ win rates: 51.86%, 50.13%, and 53.57% in that order.
Finally support, where 
combine for 98.54%. Even though 6/7 are over 50% win rate (with Janna having a whopping 53.65%), it's still nowhere nearly as bad as the ADCs.
Let's try to explain why ADC is so dominated by so few champs. Look at those other roles and notice how each of them have a variety of different classes in the top. Look at top lane for example: the top 4 consists of a juggernaut (Darius), 2 skirmishers (RIven and Yasuo) and a tank (Poppy). You can do the same for the other roles.
Then look at bot lane (the carry role, not the supports). It shouldn't be a surprise that there is exactly 1 class of champs being played there. What does this mean?
Let's go back to top lane. When Juggernauts happen to be too strong, they will never have such dominant play rates, simply because the enemy could counter with something like a Gnar.
Then look at bot lane. When a couple of ADCs are too strong, there is simply no reason not to pick them. Yes, there are champs who counter ADCs, but there is no alternative, so it's better to just go with the strongest ADC. Yes, Tristana might be slightly better than Jhin at escaping assassins, but that difference is too small to compensate for Jhin just being stronger overall.
TLDR: It takes only 4 ADCs to reach a combined 100% play rate, compared to 13/8/10/7 for the other roles, with those 4 all having more than decent win rates. The reason for this is because (unlike other lanes where there are different classes with their own weaknesses) there is no alternative for an ADC bot, and when you need to pick one anyway, why not go with the strongest one?
