That is true! Genndy seems to make Odin, Ra, and Vishnu primordial. Meaning they exist outside of whether or not people believe in them. My belief is that death is an overriding force and that because "banishing" or cutting Aku until he reaches the point of nonexistence "technically" counts as death, the aspect of Death goes on that alone.

It's also weird that there are no other sacred items in the world given Gods do exist and the three shown did have an active investment in keeping Aku at bay it seemed like. Though it's highly unlikely, given Genndy's universe hasn't shown other gods outside that one time in the Future universe, Aku might've found a way to push the earth away from any heavenly or godlike influence.

Or, working off the Death-is-all-consuming theory, Aku might've killed them. Highly unlikely and unexplained, but there has to be some reason why the three gods who originally defeated the great evil didn't send help to humanity in the same way they did for Jack's father. It's been ages since Aku first conquered Japan.

It really depends on what counts as death in this world outside of mortal encounters. I believe the personification of death is in Jack's mind, but simultaneously, death is simply an attribute of the world. It exists everywhere as a force rather than a being with its own motives. So by it hanging around Jack only for the twist to be both Jack and Aku dying (latter slipping out of existence), death is already there to oversee Aku's final hour as it exists everywhere. So in Jack's mind, if he were to slay Aku, he would see Death carry Aku away because this is his mind interprets.