Mercy was the only one perhaps aside from old Symmetra that had to play VERY passively because of the obvious reason she doesn’t have a reliable way to fight back and your best option is to usually fly away and reposition. She played way to much around her ult compared to other heroes. I wouldn’t say the SR was the incentive for the hide and seek playstyle because not many players even knew it exsisted and were doing it without knowing. It came from baiting out ultimates and just undoing them all while your somewhere safe from all the crossfire. Plus when they removed it (I think a little before the summer games last year) Mercy’s were playing as they always have SR incentive or not.
But the changes people are asking for make her already “balanced” ultimate weaker for the sake of it being weaker.
If she has a cast time plus LoS she would probably feel awful to play because in order to res you need to be out in the open with no cover to obstruct your LoS and now you need to take into account that the cast time can get you killed before you could even use it and you don’t have the option to use cover to make it easier and more rewarding.
If he said that then I’ll take your word for it but their should be better ways of balancing her. EeveeA made a really good post about changes they could make to her, I don’t agree with all of them but they are definitely a step in the right direction in my opinion: Suggestions for Mercy from someone with too much time on her 📝