This reads a bit like a throwaway line from Nanami, but it’s actually a meaningful thing for Gege to insert here.
For context, Nanami asks Geto this question after coming back from the failed mission where Haibara was killed.
This scene is also immediately before Geto's village massacre / defection scenes.
This is the very next set of panels: (It’s the continuation of the thought Geto started above, then cuts to to the mission where he kills the 112 non-sorcerers)
So, what Nanami says to Geto is the last thing we see anyone at Jujutsu high saying to Geto before he makes that final decision to turn away from being a Jujutsu sorcerer.
Geto’s thought about the “mountain of corpses” is the last internal monologue / insight we get into his mental state before defecting.
In Geto’s mind, who is in the hypothetical ‘mountain of corpses' created by Jujutsu society?
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Let’s start by acknowledging that Nanami’s suggestion to make Gojo handle all the missions actually makes sense, from a strictly pragmatic perspective. (Why let weaker sorcerers die doing something that Gojo could easily handle?)
Even before Gojo learned Reverse Curse / Hollow Purple, he (and Geto) were given the Star Plasma Vessel mission on special request from Tengen. Presumably because they were the strongest sorcerers.
It’s reasonable to assume the higher ups in Jujutsu society have thought about optimizing the mission assignments this way. And, by this point, Gojo is getting sent on missions, alone, without Geto.
But even though he’s the strongest sorcerer, Gojo is not unbeatable. He’s not even as powerful as he’ll become by JJK0+
Here’s what Gojo told Geto just days before the mission where Haibara dies:
So Geto knows that Gojo can’t always run limitless, and he hasn’t perfected domain expansion. Gojo can’t protect himself against poisons.
Geto also already knows what happens when Gojo’s power is overestimated - when he is overworked and spread too thin. Last time that happened, he got stabbed through the chest by Toji.
“Impossible!” is Geto’s reaction, because he’d been lulled into a false sense of security, thinking that Gojo was untouchable.
Nanami suggesting that Gojo handle all the curses is sort of like Toji putting up a bounty for Riko. Each weak assassin wasn’t enough to take Gojo down on their own, but en masse they could distract and wear him down.
The same would be possible with a never ending onslaught of curses to exorcise. Then, all it takes is one mistake before Gojo's corpse is added to the mountain.
Don't get me wrong, Geto had many reasons for defecting, and Nanako, Mimiko, and Riko’s treatment by non-sorcerers were probably the main ones.
But Geto must have also believed that the world he was trying to build for Jujutsu sorcerers was a better, safer world for Gojo, too.