The argument ADCs are strongest because they're in every game needs to stop
ADC is a role filled typically by marksmen because they usually require defense due to their innate squishiness and weakness to CC. The reason mid and top is filled with so many different classes is because these champions of such classes can often hold their own and can give more influence to the game in other ways. A marksmen/ADC top-lane creates a squishy composition which is easy to abuse while a marksmen in the mid-lane often lacks the range, mobility, escape, waveclear, or roaming potential of other champions.
In lane, even when an ADC was meta, such as Kai'Sa is now, the support still dictates the flow of the lane. If the enemy has a Blitzcrank and you have a Janna, you can bet that even Kai'Sa will need to play passive usually.
ADCs aren't broken, supports are just as strong - if not stronger.
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