It’s never that simple and you know it. Shields go down while Rein swings or firestrikes or teamfights, shields move, flanking isn’t necessarily up in your face, especially with the range options of Hanzo.
Plus what if you’re the flanker, going after their backline, Genji, Soldier, Reaper, Sombra, even Symmetra, you get behind the Hanzo unnoticed, but every one of them requires multiple hits to kill him, where Hanzo still has that coin-flip luck factor to land the single shot he needs at ANY range.