I would say while it is true that he will help a lot more. It is also important to remember that he is getting older and his body isn't as well as he used to be. When I say "older" people keep defending and say "but Ronaldo is 40 years old and still playing awesome?" well do you think KDB and Ronaldo has the same body?
KDB never was a runner, or go back on defense to help kind of guy, and City had a great system of playing that allowed KDB not to help out on defense too much. Napoli may not be able to afford that, it's going to be quite obvious that they are going to end up with much more issues on defense when he doesn't go to help. He will be the perfect registrar, but without anything further, they are not going to end up getting much help, just good passing, that's it. That's a huge problem when it comes to defense.
Kevin is a smart and skillful player who plays mostly with his brain and sharp skills than relies on his physical strength which in fact is not his advantage compares to other players. Now as an old player, he has more motivations to adapt to ageing, weaker body and take advantage of his smartness, cleverness and great skills as well as abundant experience in his playing time and contributions for his club.
Napoli actually are aware that Kevin can do this well in Serie A with low intensity in most of matches. They needs ball possession and match-changing passes from Kevin, and physical fight in the midfield will be assigned as tasks for younger players in their club.
It's clear that they bought Kevin de Bruyne for his vision of the game, it's clear that he's not a player who can run 100 metres with the ball at his feet like a twenty-year-old would, but it's clear that he has a lot of experience and is an excellent player who can really give a lot