Nvm, I got my answer… low rank… Do you think that Phara is faster than Mercy? Do you know the range of her GA? Do you know that Mercy has slow fall right? 
But I totally understand you, Mercys on your ranks can’t do the basic Mercy things, that’s why you think that the Mercy won’t be able to keep up with Pharah… Also, have in mind that her new passive will make Mercy harder to kill as well.
Anyway, I don’t disagree that the GA nerf is bad but not where you think. It won’t do anything against Phara Mercy duo. On contrary, it will save her Pharah easier when she is under 50%. It will do other things though. Sorry for ruining it for you but I have 848 hours on Mercy and Diamond Rank, I know where the nerf will hit her hard and where not.