Napoli must be aware of how tight the competition is between other teams next season, not only in the Champions League, which certainly has strong teams. Even in Serie A, other teams are also preparing to strengthen their teams. Of course, Napoli will prepare their team for the competition next season. That's what they showed by being quite active in the transfer market this time. They are not only satisfied with their achievements last season.
Napoli's persistence to get de Bruyne finally became a reality. After the rumors that circulated, finally, it has been confirmed that de Bruyne joins Napoli for a 2-year deal and several agreements. Furthermore, they will do a medical test and also other formal requirements. 
Source: Kevin De Bruyne to Napoli
In addition, Napoli is also doing quite massive signing in this transfer market to defend the Scudetto title. Some other players they are eyeing are: Yunus Musah, Borja Sainz, Oumar Solet, and several other players on the list.
I also wrote this under another post, I think he is a very strong player and that he was what Napoli was missing to become a top club at national and international level, considering that ADL never makes purchases by chance, he has always been a president who knows his stuff and doesn't waste money