He is only trying to learn ADC champions. Are you *****ing kidding me?
He is, by intention, only trying to learn about 12-18% of champions in the game. Likely, even fewer different types of match ups. Meanwhile, every other lane/role in this game has already understood that they need to learn all kinds of champion. You don't see Impact crying and requesting to be subbed out because tanks aren't in meta. No, Impact goes on to practice and play MORE than just tanks. He learns to play tanks, adc's, mages, bruisers, supports..etc.
Hell, even other ADC players are starting to learn that they need to know how to play other roles/classes. It's idiotic to think you should only learn to play that one specific type of champion. Can you succeed? of course! but you make it a lot hard on yourself and you in turn become a one-trick pony.
ADC's aren't dead. They just aren't a mandatory pick in competitive anymore. Like any other class, it's based on team goals/draft. I'm a big fan of Rekkles; he is still young and talented. He can(and already shows) he will try to learn other classes.
I hope Riot doesn't start overturning some ADC nerfs because of some pro's who refuse to adapt and learn. This too is something team organization can take into account; tell you ADC only players to learn other classes. Perhaps swap out lanes and let ADC's go mid. Echo Fox is clearly doing it.
It sucks that Fnatic has a mid laner that is very diverse who CAN play adc's at a level close to Rekkles. The same goes to G2, perks can play ADC; so in response, their bot lane learn to play mages. Not a hard concept.
Time to ADAPT or lose your job.