I see impossible to see Napoli being champion of the SerieA back to back, they never do something like that plus Inter needs to win something asap after losing it all in the end of the last season.
This plus this Napoli seems to be the best one and when this kind of teams are in that particular position they start to fall.
Napoli didn't play any European competitions last year and had a very advantageous fixture schedule. Inter, on the other hand, had reached the Champions League final and looked like a team that, with its tired players, struggled to qualify for both tournaments. The disappointing thing for Inter was their loss in the finals. They emerged from a very demanding fixture list but failed to win either the Italian league or the Champions League. As for Napoli, I don't know what they'll be able to do in these two grueling fixtures next year. I think their chances of winning the title are lower than last year.
Of course, they might not win the title next season, because they will participate in UCL which will make them to be engaged with many matches than what they played last season, and that will definitely affect their performance. However, Inter failed to win Scudetto and UCL last season, but they are the most successful club in Seria A last season, because they were runner up in both competitions.
Fixtures provide a significant advantage to some teams, but not every team can capitalize on them. Napoli capitalized on this advantage and were successful. I don't know if they'll have such a fixture advantage again, but repeating this success is no easy feat.
Inter and Napoli are successful teams and must work hard to maintain their success. Competing in every competition isn't easy for teams, and sometimes they have to make choices. Inter's performance in the Champions League last season was excellent, but achieving that this season is proving difficult.