If ADCs do so much dmg early, why not make crit damage scale with crit %?
Honestly, if you want ADCs to scale, this is a potential solution. They'd be weaker early but would scale well, which seems to be what most ADCs are all about anyway. Say you have 30% crit chance from Runaans, then you get +30% dmg on when you crit. Obviously, the more crit you get as you buy items, the more damage you do. 100% crit would be double dmg on crits, which is what the system is now.
Power creep has been a problem in this game for a while now, and Riot seems to be doing something about it with the Rylai's nerfs (+ reduced AP in general from Mage rework) and the Assassin rework. ADCs do too much damage early and this is a way to bring the class more in line with its role without having to nerf items or play around with individual champs.
I might also add that this would solve that cancerous Trynd lvl 2 cheese or those 1% crit runepages that Riot hates due to RNG. Obviously RNG is still present, but less impactful, which I think is healthier considering early game does so much to decide the course of the game you're in