Please do link where they state that Mass Rez was a bad design.
The developer update for the Mercy rework.
I won’t go into the whole “why mass ress was bad” discussion because I have covered it so many times but I think this is important:
Mercy’s Resurrect has always been a key feature to her. If not directly noted, it’s been heavily suggested. They have so much around Resurrect, to remove it means to have to redesign a lot of merchandise and rewriting her lore.
This is a good point, however I think at some point or another, when all other avenues have been exhausted, you have to bite the bullet and do what’s best for the game.
Every iteration so far of Mercy and res has been imbalanced. Sooner or later there won’t be anything left to change. They can try tweaking healing numbers perhaps but I suspect it won’t make a huge difference. They could try tweaking her mobility but that would make her feel awful to play. I doubt they will ever touch her healing beam either (consistent, no aim requirement).
IMO looking at it objectively as possible, disregarding anything outside of core gameplay/balance, the most obvious part of her to kit look at is res.