First, the Napoli coach is new. We are not sure if he can stabilize the team. Secondly, there are going to be several important changes within the Napoli team. Several of their key players will not be with the squad next season. Thirdly, teams like Inter Milan, AC Milan and Juventus will fight for the title next season. They are trying to make themselves stronger. And so I don't think Napoli will be favourites for the title next season. Even maintaining a stable performance will be very difficult for Napoli next season.
The beauty about football is that things can turn out rather very good for Napoli next season.
Not very many important players will leave Napoli this summer. I feel the ones that would leave can be replaced and there will be one or two additions to the squad.
Napoli may not be a very rich team but they don't easily sell off their star players. Kvaratskhelia will not leave and most likely Osimen will not leave because the price tag on his head shows Napoli are no ready to sell.
All the teams you mentioned fought for the league last season but Napoli still won the league. So them buying players and reinenforcing doesn't mean Napoli will not also compete.