No, you’re ignoring that both Ana and Moria need more skill than Mercy does. Both Ana and Moria are balanced, Mercy is just too good and they seem like trash in comparison. Take Mercy out of the equation, and you have 2 main healers, and not 1.
But I am NOT giving into a support power creep when Mercy is 10x as effective as the other supports but requires less risk and less skill to play.