I think Sean saw the long, very painful road to rebuilding this team would have to go through in a glimpse throughout the 2021 season and he decided to leave, or quit, as some cynical, negative Saints fans at the time saw and perhaps still see it. Winning wasnt going to be quite such an easier, transparent task anymore without #9 so he'd retire for a year and rest up, like Belichick's doing now (or maybe forever) and return to some new-and-improved more "ready-to-win now" team that has more talent, depth, and resources that produce 10+ winning seasons faster.