This again....: " adcs get their 2nd item at 20 minutes"
I already did the math in the last thread to show that 20 minute 2nd item completion is late. most expensive 2-item ADC build is 7900 gold (maybe add a bit for potions and a D blade in the worst case. Even 400 GPM is 8k at 20 minutes, and 400 GPM is low. Anyone who is good at finding farm should have at least 450 GPM if not more. So 2nd item is more like 16-18 minutes, 3rd item is like not later than 24 minutes or so. Many 3-item ADCs are still pretty impactful at 2 items+some components (and often their 3 items are somewhat cheaper). ADCs are arguably stronger than most other champs at 3-items and it only gets more 1-sided in the favor of ADCs from there.
Moreover, I am assuming ADC doesn't get fed, if they do that time comes down quite a bit and because they never fall off (and in fact, only get stronger in terms of relative power to other archetypes of champion) a fed ADC can quite easily just win the game for their team (why do you think permagank bot lane is such a meta strategy these days?) ADCs taking time to become strong is the counterweight to their fantastic late game. You start letting them reach late game to early and teams will start picking 2-3 marksmen every game because they're just better champions at that point.