Managing a game is like driving a tanker ship... sometimes you know you are off course but you don't stop your engines unless you are in colision course.

Mercy is dominating but Blizzard will take it's sweet time and change her on the next balance cycle. Meanwhile, the smart persons among us will pick her and reap the sweet, sweet SR.

This said, if she was a DPS, people would be very salty like on the McCree age. It also helps Mercy is super easy to use so both teams end up with one and the game end up "balanced".