I see KDB still ambitious to play in a European club at his current age, actually if he chooses money of course the Saudi League will give it, it is even possible that it can be more than what he got in Man City so far, therefore the steps he took with Napoli at this time should be appreciated because he sacrificed a salary cut just to be able to play in the famous league. I will not question his injury, it is still comparable to what has been agreed by both parties, yes hopefully his magical legs will be shown in Serie A.
Kevin is a very good player with abundant experience in this career so he will be able to adapt very well in Serie A and Napoli. Even he is smart, clever and skillful, as all other old players, sooner or later he will no longer be able to control his fitness well for high quality performances. Even having appearances will become more challenging season by season for an old player like Kevin but at least a next season won't be too difficult for him physically.
KDB will perform pretty good in Seria A, because the competition isn't that demanding compared to EPL that's rough and tough. He is a quality player and can still maintain his quality despite the fact that he is getting old. Napoli has a good coach and promising players so his presence will add more strength to the team
Whenever Kevin has good fitness and is ready for matches, his performances won't be too bad or below average quality. Playing in Serie A with low severity of physical fights possibly help Kevin De Bruyne to prolong his career for some more seasons. While he will still have opportunity to play and fight for Scudetto as well as participate in UEFA Champions League. Napoli are truly lucky to have Kevin De Bruyne in competitions with many big and rich clubs.