An ADC's job has never been to kill enemy champions. Thats largely just a side effect of them building damage. The ADCs job is to deal heavy DPS to objectives to take advantage of short death timers to close games out fast enough that the enemy doesnt get another shot at a comeback.
If the ADC and a tank are alive, and healthy, on an ace, they can usually get an objective. If the support, tank, and jungler are alive.... its usually very difficult to get a major objective before you have to reset. If dragon is up or a tower is partly down, you can grab those, but baron, elder, or a full health tower in a not fully shoved lane are generally out of the picture.