It is true that Napoli has changed their poor performance, and they were lucky enough to win the Scudetto this season, but I don't see Napoli as the favorite to win the Scudetto next season because if not for Inter Milan's busy schedule in the UEFA Champions League competition, Napoli wouldn't have won the Scudetto this season. In my opinion, Inter Milan is the favorite to win next season's Scudetto because they will want to make up for their mess next season.
Napoli only improved with performance and added some improved productivity but they are yet to perfect their consistency so I cannot trust them enough to do anything fantastic in this new season, I will rather anticipate Inter Milan doing better than hoping Napoli will be able to defend the sucdetto title. It is even possible we get to see Juventus do better then them, Juventus is making some moves that looks like we should raise our expectations but they suffer the same fate of inconsistency with Napoli.
Since Conte became Napoli's manager, many positive changes have occurred, even in his first season alone, Conte managed to deliver the Scudetto for Napoli. So I don't think there's any reason to doubt them next season. Every manager likely develops their team better over time. Their first season was already very good, and there's a chance they could improve even further next season.
In fact, I doubt Inter Milan next season. I don't see them making any positive changes by bringing in an inexperienced manager; in fact, the opposite is possible.