The last time Mercy had Mass-Resurrect, she was D-tier. The last time Mercy had no personal invulnerability on Mass-Resurrect, she was F-tier. The reason Resurrect is so strong right now is that it’s on a cooldown ability. Resurrect wasn’t nearly as powerful as an ultimate, especially before it’s invulnerability buff.
Effectively, the proposed rework takes the form of Mercy that was D/F-tier, nerfs Resurrect further, and adds a 33% damage debuff to a single target with a 40% uptime maximum (technically 38.1% maximum uptime if you factor in the .5 second windup).
I will say that the listed charge requirement is now outdated, seeing that ultimate charge rates have been nerfed across the board since I first posted, but I’m not going to change the proposal for reasons outlined when I contextualized it. That number can be easily changed if this proposal were ever applied anyway.