Let's say Keystones did increase Damage
Is that a bad thing? Is it a bad thing that, when somebody manages to unleash their QWE or peppers you with AAs, you can't just shrug it off? Why is rewarding consistent aggression a bad thing? Is passively trading CS really the best iteration of LoL gameplay?
For what it's worth, I disagree. I think at most, Keystones evened out the damage done over the course of the game, i.e. more damage early-mid game, less damage mid-late. The old masteries had the problem that either to early-mid game oriented champions ran rampant to make up for the lack of early-mid game damage ... or that everyone played for late game (good old S5, you won't be missed).
EDIT: Who needs arguments? Downvote and move on.

