Definitely Napoli have a very good potential. This season, like Milan, they have the best defense, conceded only 31 goals, they also have excellent attack, they scored 5 goals more than Milan. But they lack the stability to have a season without recessions. Perhaps the team really lacks the mentality of a winner, but for this the team needs strong leaders with such a mentality.
Any team at a distance will lose to a top club for only one reason - squad depth/reserves. In almost every season, you can see a couple of bright teams that overtake the leaders but by the end of the season experience big failures and give way to leadership (exceptions like Leicester happen but very rarely). And this is only a matter of resources and not mentality. The mentality decides in knockout games.