I think you’re missing the rationale behind nerfing her. I, personally, didn’t think he needed a nerf, but they aren’t looking at the game from my perspective. That said, from my perspective, I just switched off if I was getting countered by him.
This is not a good way to rate dps, which I think is a major flaw in the stupid scoreboard. What matters isn’t damage or participation in kills. It’s getting the right kill at the right time that turns a game. DPS are designed to do this. Moira? You start throwing if you go for this too much because it takes her cooldowns and time away from healing to even attempt it.
This isn’t really related to Genji, so I won’t go too far into it, but I disagree with your assessment for a few reasons.
I don’t know what the WR is for Genji vs McCree in GM, but again, their perspective isn’t who to nerf in deathmatch. It’s who is winning more games for their team and how.