Why ADC is a problem
Toplane: Tanks Bruisers Splitpushers Lane-Bullies
Mid: Assassins Control Mages
Jungle: Tanks Supportive Farming Counter junglers Assassins
Support: Mages Enchanters Tanks
ADC: Marksmen
Every other role has different options that work in different ways that are always available. To elaborate, if assassins suck, mid players can fall back onto control mages. If tanks are bad, toplaners can swap to splitpushers or bruisers. If marksmen suck, 1/5 of the players have no recourse and are forced to play through a game that will not be fun, and if they end up in a meta like this one, where you have 2 tanks to peel, a janna with shields and a control mage to cc incoming players it feels pretty good, but not for other players, and not for the losing ad carry.
As you can see, a difficult problem to balance. If they are strong, 1/5 of players is having a great time, and if they arer too weak the opposite is true, and neither are healthy.
In my opinion, a new class is required to give botlaners a different option in champ select, and allow marksmen to fill more of a niche. I don't think that constantly raging against 1/5 of all players will produce an outcome of any value