This. The ADC update focused on making marksmen "feel" different (which they somewhat succeded at; Graves is less generic now) but it failed at looking at the class as a whole and its purpose in the game.
Think about it. What is an ADC's main purpose?
Miss Fortune: Deal damage.
Lucian: Deal damage.
Jinx: Deal damage.
Twitch: Deal damage.
Caitlyn: Deal damage.
Corki: Deal damage.
Now, we have some exceptions :
Ashe: Deal damage, stun people, slow them.
Jhin: Deal damage, snare people, zone them.
Sivir: Deal damage, press R.
Now compare this to another class. For example, supports:
Nami: Stun people, speedboost allies, heal them.
Janna: Protect her allies, peel for them.
Lulu: Protect, help an initiator engage, disengage from assassins by turning them into furries, poke.
Bard: Roam/gank, provide his team with mobility, do whatever he wants with his ult.
Perhaps you could say I oversimplified every ADC's function. But we still can't deny they all offer mostly the same, which is the reason why there's almost no variety in said class.
Riot could maybe fix this by giving ADCs more to do than just deal damage. Give them more utility, perhaps, like Ashe and Jhin have. I don't know. Maybe I'm talking out of my ass since I suck at that role. But I love talking about game design and I'm sure this is the main issue.