It's hard to say that Napoli would have successfully defended the Scudetto without Spalletti as coach, because I have little doubt about Rudi Garcia. On the other hand, the fact that one of the factors that Napoli succeeded in winning the Scudetto was due to the declining performance of several clubs that often lead the Serie A standings, such as Juventus, AC Milan, Inter Milan.
For now, all the teams in Serie A have made a lot of changes in the squad, AC Milan has been the most aggressive in recruiting players. Of course Napoli will have a hard time defending the Scudetto, unless they have great luck.
Rudi Garcia is not new manager in Serie A competition, he has know well after long time as AS Roma manager and know how difficult how to defense scudetto next season, but I don't doubt with Rudi Garcia capacity actually Napoli squad not change drastically from Spalletti era except losing Kim Min-jae. I don't see any drastically change with Serie A teams because several new sign not regular player position current AC Milan, Juventus and Inter transfer. Optimistic with Napoli has chance for defending their Scudetto next season due Victor Osimhen still in Napoli squad and they have more solid player than rival teams.
Usually, Juventus always have surprise with transfer activities but for season 2023/24 there are not any significant transfer did and difficult for signing Lukaku.