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.