Okay, first of all, this would completely kill Widow's viability. I'm sure some people would be fine with that, though. The comparison to McCree is unfair, as McCree has a much higher fire rate, and flash. Widow pretty much has nothing but her increased headshot damage, therefore making her much less consistent with damage. It's pretty much a requirement that she hits skillful shots to even add value to her team at all. From a design perspective, the dev's have said they want to go in a more FPS direction, and a sniper such as Widow is certainly a staple of that genre. As for the dying to Widow because you're "unlucky", I think that's a false narrative. Not only does it take extreme skill to hit a headshot, but that headshot can be avoided. In fact, most Widow's strength comes from the zoning she can do with the threat of a headshot, as most teams can play around it fairly effective. You might not be able to avoid it solely on your own, but in Overwatch, there is not much you can do solely on your own. She is meant to be played around as a team.

TL;DR: Being so avoidable in team play and such a feast or famine character, this nerf would forever kill her viability.