People like Mercy nerf cuz they only like something with killing the whole team and bring massive destruction effect, so they hate if someone get in their way. Its Mercy.
If you agree with Mercy nerf, theres nothing different between Overwatch with other FPS game like COD.
Thats why they design Mercy to make the game special, why its offended you when someone can change the match with different way except killing?
I like to play Mercy cuz she can resurrect teammate and turn the table without just ''Killing'',she is special, so now everyone just push her away, and thats means she is not a good thing for most of the people.
If you dont like something, why you let them in and torture them ? Why they design Mercy ?