Napoli is not a favorite at all for the next season. They had a good performance this season and they won the title but since their coach and some players are losing this team I think they don't have too much chance for the next season.
This is not a certainty. They might still do well.
Players and coaches leave club all the time, that doesn't mean that club has to be miserable. It all depends on how they handle their affairs. Napoli will lose a few important players, but Im not sure it would be more than 3 or 4. They can easily buy some replacement. Buying players should not even be a debate for Napoli. Whether their players leave or not, they have to strengthen their squad.
Osimhen might not leave. Napoli placed a 180 million price tag on him.
If we pin that Napoli's best game happened to their coach this season, then maybe they won't be better than the season they managed to conquer by winning the Scudetto title. But football is not like that, meaning anything can happen, even if they are still with the same coach next season.
This depends on themselves and their own performance, Napoli's solidity this season is not luck, or in other words they really prepared everything so that they managed to be a differentiator in Serie A this season. They should have known what they had to do with a lot of studying in the previous season.