I tend to agree with you on this point. Napoli played fantastically in the beginning then dropped a little and continued fluctuating until the end of the season. This all happened while having their best players and sometimes after losing them to injury. I think the last point show that even with the full squad, Napoli might lose its title.
Next season I see that Inter Milan is a strong candidate to win the Scudetto because their squad has seen good perfection this season. Maybe Lukaku will return to play from the start of the season next season because last season he was hit by an injury which prevented him from playing fully in every game. Martinez and Lukaku are the stickers that are very feared in the next season. Dzeko is gone and he has made a big contribution to Inter this season.
Napoli are not too strong in the next season if Napoli are left by their core players who go to other clubs in this transfer window. I think they are a surprise team and it will be difficult to defend the title next season.