ADCs are the strongest champions in the PVE aspects of League, taking towers/dragons/barons, etc, while people only care about the PVP aspects because PVE aspects aren't fun, no one has fun just standing still right clicking an objective and watching the damage roll in.
Even when ADCs are weak in PVP, they still have a lot of value because of their strengths in PVE, and it takes ADCs being really weak like in patch 8.11 for it to actually be suboptimal to run one. Just because teams are still running ADCs doesn't mean they're not weak, just that their weakness in PVP isn't enough to compensate for their strength in PVE.
Even though making ADCs balanced in the PVP aspect sorta makes it so they're overpowered because they're both the best in PVE as well as equal to anyone else in PVP, it doesn't really matter too much because a) you can't really run more than 1 ADC per comp anyway without having a bad comp, b) the fact that both teams have an ADC cancels out any advantage gained from ADCs being so strong.
The PVE aspects of ADC also don't really take any skill at all, only when they're combined with PVP interactions, so the PVE advantages really are completely canceled out when you have an ADC on both teams besides the fact that some ADCs are just better than others are at sieging/taking neutral objectives.
The result is that the only strength that really matters is PVP strength, both from an enjoyment perspective and a balance perspective. So long as their strength in PVE isn't further buffed or their strength in PVP overbuffed, to the point where it's smart to have as many ADCs in your comp as possible because the positives override the negatives, it doesn't really matter that ADC's PVE strength over other classes makes them technically better than those classes, because the PVE strengths are a strength no one enjoys exerting and takes no skill to exert anyway, and so long as ADCs are balanced to the point where they're equal rather than superior to other classes in PVP, it won't make ADCs start taking over other roles, except in cases where those ADCs are more at home in those roles than on bot lane (jgl Graves, top Quinn, mid Corki).