Well what else would she do if she wouldn’t be resurrecting. Letting her teammates die. Of course not.
Mercy 1.0 didn’t encourage them to stop healing, that is what some players started to do. Mercy could keep them alive if she didn’t stop healing. Less Mercy players actually used to hide and rez than you think. Most of them didn’t. It’s just a myth. Anyways, even if she was hiding to stop healing and then go in an rez after the enemy ulted, how is that any different from a DPS going on a flank to do their ult. Both times they aren’t doing their own job, and when they do it would probably be impactful.