Pretty much this.
Pre-invulnerability, Mercy had a suicide rez problem. If she made any rez bigger than a solo, Mercy made herself the center of attention with no way to quickly leave the area.
Enemies who had previously tuned out mercy’s presence (because of the “if it’s not shooting at me, it’s not a threat” FPS mentality) would suddenly be reminded that Mercy is not a harmless NPC, and take out their frustration at the turn of events by killing Mercy in a vengeful rage.
Since Mercy didn’t have the bunnyhop or superjump, she had nowhere to run after she rezzed (which put a huge target on her back in the process).
I think that adding the GA tech we have now instead of invulnerability would have fixed the “suicide rez” problem in a smart way. Dumb mercies would have still paid for making mistakes, but smart mercies would have survived more rezzes by planning ahead and managing their GA tech to escape enemy focus after a rez.