Then we should make all of junkrats grenades not do damage if its out of line of sight, stuff happens in these games. Same should be applied for Hanzo, I would not say get rid of his one shots, but make his projectile smaller so its not a log sized thing that just kills people.
This argument of dying to oneshots and not having a chance to respond just does not seem fair, thats like saying you should be able to respond to a tracer blinking into your face and sticking her pulse bomb on you. What if you snuck behind the backlines and lined up your shots to kill the squishies, why should you get punished for taking the time to flank and get the kills?