If De Bruyne avoids injuries, which is a big if, for sure he would be one of the best Serie A players despite the age. I'm not a Napoli supporter but I have to admit that it is a great move, he can really make a difference, especially in the big matches.
Old players are more sensitive with injuries and De Bruyne is not an exception. He will become more sensitive with injuries and get more injuries with time together with longer recovery time after each injury. In Serie A, match intensity is lower than in Premier League and it can be helpful for De Bruyne to adapt to this league and prolong his player career.
With his smart brain and great passing skills, he will have more time of finding defensive holes before making sharp passes for his teammates and it will possibly his main contributions for Napoli.
This season is going to be very exciting and thrilling for the scudetto. As every club is going to have better and best to win the scudetto. Everyone is struggling but the Milan, and Napoli team is going to have a tough competition. Because both are signing the new players to make the win possible. The players they signed are an extraordinary step taking by the team. But the new players of both of the teams KDB and Luka Modric are not so favourite by some of the viewers as according to them this competition is for the young players.
Both AC Milan and Napoli truly knew and expected that they can use old players like Modric and De Bruyne appropriately when match situations allowing them to use those players. Old players with great skills, abundant experience and strong mentality to deal with pressure can be very useful for clubs if they are sent in at appropriate time during a match when they can face with less physical fights with younger players while can use their experience to support their teammates better.