Having Rez as it is now makes it as powerful, if not more than Mass Rez because of the sheer fact of frequency.
Mass Rez was limited by earning, as is every other uniquely behaving ultimate in this game. No one else has it.
By simply introducing another unique method of bringing back someone from the dead, it would have negated peoples obviously biased feelings toward Mass Rez.
If it is not okay for Mercy to have a unique and powerful ultimate ability, then no one else should be able to either. The fact that every ultimate has the potential to impact multiple is enough to make Mass Rez entirely fair and always has been.
Simply adding a mechanic to earn each individual rez for a Mass Rez could increase the skill floor of it. Each rez costs X amount of charge that you can earn, it adds up and you can choose to use when you only have 1,2,3,4,5 charges worth but can use it even while everyone is alive. Any charge that is unused for resurrection can be used as pulses of burst heals but not in large numbers, like 50 instant heal per pulse. It means Mercy can use healing to earn more healing or choose to resurrect. It adds a much more interesting dynamic and every individual pulse/resurrect is earned and stored.
EDIT: I am actually going to back pedal on the 50, I want the pulses to happen rather quickly so let me drop that to 25, so that after like 4 pulses for example, it should heal squishes but not much on tanks, so you have to do that manually to save anything.