Things are a little different with ADC versus top/mid/jungle though. Many of the champs fit a very similar role into a team comp, and there's less of them that are relevant (or 'meta') at any given time.
Sure, Turtle could learn a champ like, lets say Jhin or ziggs, that plays differently, but if Jhin isn't in a good spot for the meta it doesn't really matter. It's arguably more important that the support have synergy with the player, and the team being on the same page as the ADC and support, then it is which specific champ they are playing (within reason, i.e. not playing a F tier champ like teemo or something).
SKT has been one of the few teams I've ever seen pull off multiple subs and it not really effect how the team plays together, and to pull off different strats based on that. But, they have never done it with ADC/Support and I think that's very telling. That lane is all about synergy with your teammates and being on the same page, moreso then a potential cheese strat. Even when someone does something different, like the Ziggs split push strat, it requires the rest of the team to execute it. Whereas a sub mid laner could have a pocket pick that dominates his lane and then just hard carries early on.
I mean, imagine if TSM had a Top sub this split and had been rotating them in/out for random games. Would hauntzer be at the same level he is right now? I doubt it. He'd still be good, sure, but not 'potential mvp' like he was.
I think Turtle is mostly there to light a fire under Dlift's ass to make sure he doesn't get complacent again. Whether Turtle ends up being ok with that or not, we will see. If so, and he doesn't mind playing 10-15% of the games, he'll stay. If not, he'll get traded to someone else.