They will be in the Champions League next season, and it will take work. But actually they have experience this season when they can go to the quarter-finals of the Champions League, and they have to be beaten by fellow Serie A representatives, so they have to be able to improve when they are in more prestigious European competitions.
I don't think there's anything to worry about too much because after all, previous teams like Inter and Milan who previously won the Scudetto but then they also couldn't maintain their performance, right. So yes, if Napoli's performance declines next season I don't think it's something strange and or surprising either because after all, such a decline in performance will definitely happen. But even so, I personally will still believe that at least Napoli will still be able to compete in the top four or top six, because most importantly Napoli's decline in performance has not made Napoli plunge into the relegation zone.