Superman is immune to kryptonite in the later comics. And yes Goku gets stronger, but like I said, how can he reach something that is limitless? Goku has limits, which is why he has died many times in the anime and constantly been brought back to life. And he has reached the limit of freeza, and cell, and majin bu. But when he goes against something that has no limits, then there is nothing to surpass.

Superman has limits. The later comics of superman arnt officially part of the storyline. Such as sneezing and blowing away universes.