If they aren’t nerfing her even though she’s one of the top 3 played heroes, what makes you think they will nerf her for being “one of the top picks” lol. Jeff specifically said in the Mercy Nerfs Dev Update on Youtube “We Want Mercy to Remain VERY viable”.
Mercy’s consistent heals slow down the game somewhat while also keeping heroes like Widowmaker, Bastion, Mcree, Junkrat, Pharah viable whereas without her they wouldn’t be very strong. They NEED Mercy to be viable to keep the meta at least somewhat diverse. Nerfing her brings us back to either full tank meta or full dive meta, which they don’t want.