The usual explanations don’t make sense. In terms of judicial punishment, it doesn’t make sense for someone to be punished for the shortcomings of others. In terms of paying a debt, if Jesus was an incarnation of God, he would merely be paying a debt to himself.

A better explanation might be that, unlike almost all the people prior to Noah’s flood who simply needed to be wiped out, some of the subsequent population was salvageable. To bring about their salvation, God’s incarnation as Jesus and subsequent death was necessary for a couple of reasons.

1. The story of Jesus’ existence as a human embodiment of God, publicly noteworthy death and subsequent resurrection was needed for Christianity to spread.
2. In order to redeem ourselves, we need to follow God’s example. For certain human experiences, there is no parallel with God. Therefore, if order to provide an example it was necessary for God to have a human experience.