Mercy being bad or good at its new role is so besides the point. It wasn't going to stay what it was, it was impossible to balance because it was inherently toxic to a simulated projectile game. For such an instant death unit to work you need hitscan mechanics where the intended counter for said unit instantly obliterates it.
It can be given smaller adjustments to fit the new role because it is now healthy for the game in said role, whatever the adjustments need to be.
Don't get why people see more ambitious changes than changing a few lua stat lines and are immediately up at arms because the first implementation isn't 100% perfect when it's working to remove a poisonous aspect of the game. Especially when said unit wasn't even necessary for Aeon to have an extremely strong t2 air stage in the first place.