I’m going to link back to this very well written post about “hide and rez” It was relevant then, it’s still relevant now: "Only bad Mercies hide & rez"
Hide and rez was the playstyle everyone encouraged you to adopt, Mercy mains encouraged other Mercy mains to do it, your team always encouraged you to do it, I recall the time where if you didn’t hide and rez you were considered a bad Mercy.
And why wouldn’t people use it? It got results, you got massive amounts of SR, it netted you an easy PotG, it gave your team a massive advantage in team fights, you could build it back up again incredibly fast (back when I mained Mercy my record was 30 seconds) and it was incredibly difficult to counter.
This wasn’t some fringe scenario overblown by some cackling villain with a plan to ruin the lives of every Mercy player, this was how you were supposed to play Mercy. Did you have to hide? No you didn’t but there was certainly no downsides to hiding.
Could Mass Rez have been salvaged? Maybe it could but Blizzard didn’t seem to think so and as such they have no plans to bring it back, so trying to justify an ability that won’t be coming back seems a little pointless.