I’m not arguing for bring mass rez back but I disagree with you on this, you don’t have to completely cancel something to counter it. Canceling the value of something is counter-play; mass rez had alot of counter-play and imo it was balanced.
Mercy’s team took 2.5 seconds to spawn in - during this time they were completely stationary an easy counter was simply to use an ultimate like Junkrat’s, DVa’s, Mcree’s, Rein’s, Reaper’s, Mei’s, pharah’s… etc. Using an ultimate to counter an ultimate is perfectly valid - e.g. you don’t cancel dragon blade form being used with Trance, but you cancel out all the value so we call it a counter.
Other forms of counter-play existed too, even though it was hard you could kill or block Mercy as she flew in, you could bait the rez by staggering kills, and you could target Mercy to either force her into a position where she could be caught in the coming ult or outright killed.
Killing a support like Lucio, Zen or mass rez Mercy before doing your team-wipe ult is counter-play. You just removed the enemies ability to counter you and have a high chance of winning the coming fight.
Mass rez wasn’t removed for balance reasons, Mercy was pretty balanced at the time of the rework in pick rates and win rates. It was removed for game-play reasons, people found it much more disheartening to see their ult team wipe and have it reversed than their ult being cancelled by beat or trance, additionally the hiding factor was a game play element that wasn’t the best.