Even if Spalletti had stayed (or any other coach had come), repeating the championship with Napoli would be a very difficult task. Napoli has been one of the big four in Serie A for a long time and is almost as good as Inter, Milan and Juventus in terms of finances and opportunities, but still in this campaign they are eternal underdogs and can take the title either against the backdrop of a decline in the rest or due to a phenomenal series victories like last season. But it’s hard to expect that they can show two such seasons in a row.
Finally someones says this, too. I can't understand how all the experts here really think that the temporary decline in their performance is due to Spalletti having left the club, that's so bullshit...
My take is that Spalletti also left the club exactly because he is a smart guy knowing that this season can never be the same or better than last season. The probability is close to zero for that to happen. That's why he left when Napoli was literally at its peak. The last season was indeed unusually excellent and when there are 16 wins, 2 draws and 1 loss after 19 games, that would have even been amazing for Juventus Turin when they were at their best.
Spalletti chose the right decision since any result after the peak will be perceived as a decline. And by the way, many coaches do this, Mourinho also did not try to win the Champions League with Porto two times in a row and went to Chelsea. There are many examples when a team has jumped into overdrive and the coach leaves it for a stronger/stable one.
It's funny that although things have improved for the Italian national team after the victory over Ukraine, there is still a possibility of not qualifying directly, which would be a resounding failure.