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.
He's quite old, but he's still a very good player. Players aged 33-34 generally prefer to transfer to easier leagues, and Napoli is currently one of the clubs with the highest expectations. Furthermore, Serie A is a league that's constantly improving in quality. As you said, if De Bruyne stays injury-free, he'll be a great addition to the team. I think it's a very good signing, and I think he'll fit in quickly
