#1… ana requiring skill to be effective in a game liek OW was a design flaw to begin with. A game that can change at a moments notice…yeah…lets have a healer that cant be flexible on the fly exist :|…
#2 SO instead of buffing/changing others you want to beat a scapegoat?
#3 Rez should ALWAYS of been an ult… but it is also not “ultimate” as it is (one every 30sec) and would require multiple charges, multiple/mass (would cause more salt than ever seen), or have her ult charge MUCH faster than it does atm.
tl:dr
Why is it “nerf this one” instead of “buff/change these up to this one” 
Mercy WAS DESIGNED to be simple, yet effective.
She was THE core healer. She didnt bring support apart from healing and to a lesser extent dmg boosting) where as every other healer had a form of support or was actually a dmg threat (zen specifically)