I don't think De Bruyne is a bad move for Napoli. Before anything else, it is a free transfer. And as far as I can see, he has accepted a huge pay cut in his salary as well.
He was earning 24 million euros per year at Manchester City but now it is going to drop down to only 6 million.

Yeah he is more prone to injuries recently but his talent and experience both can still be quite useful.
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.
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
The difference between Serie A and the Premier League is very different so I also think that Kevin De Bruyne can play more freely with Napoli in Serie A with a slightly slower tension compared to when Kevin De Bruyne played with Manchester City in the Premier League.
If you look at it as a whole, of course Napoli is very lucky to get Kevin De Bruyne because the transfer fee is free and Kevin De Bruyne is willing to have his salary cut, this is a big advantage for Napoli because the possibility of Kevin De Bruyne being able to contribute to the team is very large.
I don't think that Kevin De Bruyne is old, he is still 33 years old and in terms of quality he is still quite good, especially with the talent he has, I think it is very likely that he will be very helpful for Napoli.