Even though Napoli were ideal last season, they are still not one of the clubs that can be consistent, because competing in Serie A requires a long lasting mentality and reputation. For example Inter, Juventus and Milan, even though they lost last season in the race for the trophy, this season they still exist and in any situation in Serie A these clubs have become an inseparable part. After Inter failed to extend their trend in the UCL, this made them even more focused on locking up the trophy this season. So I'm sure Inter, Milan and Juventus will secure UCL tickets next season.
They are just going through a phase due to the change of coach they had at the beginning of the new season, it would have been a different story and they probably would have been challenging for the trophy this season again. Juventus have enough talent to be consistent for 2 or more seasons but the change of coach brought about a different mentality and it is now like they are starting all over again. I didn’t expect them to drop too much in performance at least they could have maintained a top 4 position even if they couldn’t win the title.
Serie A have been a challenging competition recently that we have seen different teams win the title and it now like no team is winning it in a row. They need to adjust their performance else there will be no hope for Napoli again next season also as Inter Milan and juventus will still be among the top contenders for the title next season.
But you aren't saying that the change of the coach is the only reason why Napoli turned from a deadly winning machine into a toothless tiger, are you? In other words, wherever Spalletti is going, that club will win almost every game in the future and as soon as Spalletti leaves that club again, then the club will turn into a losing team or an average team?
I don't know what the presumption here is, but is your point based on data or evidence? Or do you just suspect that Spalletti is a magician who through his magic hands and words turns a team into a dominant Serie A winner and without his magic, the same team falls into shambles?
It's a big claim, but if you can come up with a good explanation as to why you think this holds true, I would be interesting in hearing your point in more detail again.