You need to think about it from a design standpoint though, Tracer bomb is literally a bomb that sticks to one entity. Tracer is a flanker who isolates targets. While you “can” use it for more mass kills, say to support a Grav, the normal purpose is for single target kills.
Combining these two points into one, the ultimate does not support it, but the game itself does. The high of PoTG is a commonly cited reason among Mercy mains for mass resurrect, as well as the feeling of being “impactful.” There is impact, but not in the way that is good game design. Resurrect is easily charged up as Mercy due to her perpetually healing or boosting someone at all times. It is not hard to generate. If you compare this to something like Lucio ultimate, or Supercharger, the difference is night and day. Meanwhile, while the fact that something like a D.Va bomb or a High Noon “can” kill someone, that line of thinking is flawed. D.Va bomb is easily nullified by any shield tank or a map with walls. High Noon, Riptire, etc. all leave the characters very open to being killed. Meanwhile, Mass Resurrect can be done with no penalty because either it goes off and you get the resurrects or you die and keep it for next team fight. If the fight is near your spawn it’s even easier to pull it off since you can go out one of the other doors. Regardless of what you think of Valkyrie the reason why it is a better designed ultimate is because there’s actual counterplay to it. You can stun Mercy, actually kill her (since she isn’t invulnerable), and most importantly, the ultimate can actually fail to get value. You can completely blow every other ultimate in the game and get nothing out of it, but if you resurrect, you are always receiving value. The only other ultimates that arguably have this functionality are Lucio ult due to receiving his own shields that disappear very quickly (note he can still lose the ult due to death), Brig with her armour (that can also be stopped by killing her), and Zen, who does gain true invincibility but potentially only for himself. Every single ult in the game outside of Mass Resurrect and arguably Transcendence has a way to fail.
Where is Mercy in the team fight to kill with the ultimates if Mercy is not with the team? You can’t kill what isn’t there.
What is the actual counterplay? Mercy is invulnerable, you cannot stun her out of it when she starts. The only way you could stop it was killing or stunning her before she got into the radius, which is not easy or even feasible on certain maps, especially since it let you resurrect people out of view.
As for cast time, you’re arguing for a nerf to your own character. Cast time does not matter if you cannot die during the course of the ultimate. You would need to remove the i-frames to make mass resurrect even come close to balanced, as well as add proper LoS checks.