Crit shouldn't be for scaling ADCs
So simply put, Crit is probably the worst way to scale Marksmans in league of legends, let alone any game. The reason for this is the very nature of Crit in general. So let me get into the details as to why I believe this.
Crit in the vast majority of MMOs and RPGs would be a stat given more heavily to assassins or characters who wish to deal larger amounts of damage in their short windows of it. In league, the role of crit stays almost the same but the usage of it is vastly different. While one is just trying to increase their power in their damage window, the other is using it as a means to be just generally more power across the board. Crit is a traditional assassin stat but it is primary focused on a class who is mention to be a stream of damage.
If Marksman are meant to be a calm river that will break down the rock eventually, than Crit is the sudden down pour that floods the whole god damn town. Crit needs to massively be overhauled and it's power needs to placed else where, while giving Marksmans the tools they need to be the calm river they are inherently designed to be. ADCs with burst are not a problem in my mind, but when mixing that with Crit, it shows the main issues of the system.
TL;DR - Just replace crit.