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.
It is true that Juventus is serious in this season's transfer window about recruiting players, but I don't trust them yet to challenge for the Scudetto next because their performance is declining slowly in the Serie A league, which will make it difficult for them to rival Inter Milan and Napoli for the Scudetto next season. The race for the Scudetto next season is still between Inter Milan and Napoli because they both have a good squad depth that can consistently win matches in the Serie A league next season.