I want Mercy to be a main healer again as much as the next guy, but there’s a massive obstacle in the way: E-Rez.
A main healer with a rez on cooldown is too good not to have. We’ve seen that from the Moth meta, even when they nerfed every other part of Mercy’s kit into the ground she was still a mandatory pick.
… the only reason Mercy is ‘balanced’ now is because she isn’t a main healer anymore.
Mercy can either be a main healer, OR she can have rez as a cooldown ability… But she can’t have both. Not without turning into a must-pick (again).
If Mercy is to get a healing buff that restores her back to “main healer” status, then Rez can’t stay on her E. It takes up too much of her power budget, holding the rest of Mercy’s kit back… like a millstone around her neck.
“then why not delete rez from Mercy’s kit altogether?”
Because the devs have backed themselves into a corner with this thing, that’s why.
The devs have established multiple times that the pillars of Mercy’s design are "her strong healing, her mobility, and rez". Ressurect is featured in many of Mercy’s cosmetics, and most of all it’s featured in nearly all Mercy-themed merchandise - and Mercy is Overwatch’s marketing golden goose.
For better or for worse, Rez is an integral part of Mercy’s character design. It can’t be removed from her kit anymore than Hanzo’s bow can be replaced with a gun.
By process of elimination, that leaves us with only one remaining path forward:
Making Mercy a main healer again means that resurrect would have to be taken off “E” and moved back to Mercy’s ultimate.
There’s no other way for it to be balanced.