The other clubs are going to get stronger. I hope that Napoli will also be able to do that because otherwise, it is going to be really hard to keep up with the competition. As you mentioned that a lot of the players are going to leave Napoli. So it is going to be a tough job for them to find replacements of those players. Now maybe some players will stay in the club because they are currently the champion of Serie A. But at the same time, it is more practical for others to actually leave the club because they are already being approached by bigger clubs.
There will be a lot of change gonna happen in napoli. Spalletti already confirmed leave from the club in summer as well as min jae. That means if napoli will have different performance. You can't expect the new coach to have the same performance as previous coach caused by the new coach will have new strategy, line up and many things to be done to the napoli. in my opinion if napoli may not able to defend its trophy in upcoming season. Some clubs are become even better from this season.
Intermilan may have possibility to win scudetto next season as long as it will be performing with the same line up like this time.
I doubt if napoli will able to have perfect performance like this time.