The only way for Mercy to survive is if she stays close to a Pharah on her own team.
However, the meeting was interrupted as they were attacked by Talon forces.
Rocket Barrage has such immense damage output that the tail end of the duration was often more of a liability than a benefit, especially with fewer barriers around to blast through.
His quick damage falloff lets Pharah remain healthy and repeatedly fire rockets at Reaper.
While not as fast as Hanzo's arrows, his orb attacks along with Orb of Discord's damage magnification can force you to find a cover, as she can receive high damage from him and his teammates.