Let's set up a scenario:
Riot announces that they really want to open up tank play styles, get them into new and different roles. One of the primary ones they are looking at is mid lane. And I'm not talking about Galio, or maybe your occasional AP Malphite who builds a bunch of AP health, I'm talking straight up tanks: Ornn, Sion, Maokai, all of them building just like a normal tank.
Now, who does tanks naturally counter? Mages. You can't very well burst someone when they have 4K HP and a bunch of MR. So now mage mains aren't too happy. They don't like that they are getting forced out of their main lane that they have. But everyone just goes "Oh, but you can still just play mages in mid lane! What's the big deal? Why do mages have to be played?" Obviously, you know that that's not true, no class is required, but it still feels really shitty when your class that you love to play is just going to be made completely irrelevant.
Now let's switch this to ADCs. Currently, there are two main reasons to pick an ADC: tank busting, and safe tower pushing. Squishies kill other squishies just fine, so that's not a priority reason for them.
As Riot has stated that they want to get other classes into bot lane, they have to improve that class to do one of those two things. BUT, Riot is also working on making that both damage classes, AD and AP, can push towers just as well, most notably with the recent changes to auto attacking towers that they made in this patch.
So that leaves tank busting. So, Riot makes a group of mages (or even an item for it) better at killing tanks. What is one of the hallmarks of mages? Good utility, waveclear, and AoE abilities. So if there's a class, that is good at killing tanks, has at least mid-tier waveclear and utility, and can push in towers (oh, and btw, this class has also been defined as mainly self supportive, so they don't really need a babysitter role to help them survive), why in the world would you pick a class that can't do all those things?
That's the problem I'm trying to illustrate. If Riot succeeds in moving other classes into bot lane, then there is no real reason to play ADCs there, as the other class can do everything they can do (granted maybe slightly worse in some aspects), but while still having their own strengths.
You know how shitty bruisers felt before conqueror? When they couldn't play in their only real lane, since the tanks they were supposed to be able to fight just shrugged them off? That's the potential problem facing ADCs. They aren't welcome in other roles. Yes, that could be a symptom of them being the only class allowed to go bot lane, but I don't expect that attitude to change when they aren't.
I'm not asking for ADCs to be OP in other roles, or to take away the diversity in those other lanes, all I'm asking is that they be actually playable there, and not just a more valuable minion to all the assassins, mages, and fighters.