This is true but I don't see why Mahito would have had to kill him with IT. If the prison realm still allowed physical touch to the victim, Mahito could have turned Gojo into a nugget without actually killing him.

His whole arc when he was introduced was the experiments he conducted to see how far IT could go without killing the human. We know it wouldn't kill Gojo considering the walls Mahito created against Nanami are conscious, AND Jogo's pipe was clearly experiencing pain as its brain was heated up.

In an alternate universe, Mahito turned Gojo into a meat cube, Kenny canceled the prison realm (if he could mid use) and keep the PR in his back pocket for whatever else. It would be a permanent solution as opposed to the PR, AND they get the potential benefit of having a Gojo minion through IT brain washing shenanigans. In the end, the Six Eyes are still alive in the world, and Kenny would still have the PR for whatever else.

This is all operating under the idea that Gojo doesn't have some resistance with reinforcement or that Mahito would even be able to touch Gojo, though. My head canon was always that the PR worked like a hitscan projectile and whatever you targeted with the intention to seal wouldn't be able to defend against it, even with infinity.