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.
That is not an easy thing to get neither, you need to be a team that has been here for a while, and many times, or at least have players who have done something before, or have a team that has been playing together for a long time. Usually smaller clubs who find a gem in the rough and have a great season will end up selling their players for a lot of money which they really need, and then next season they won't have those amazing players.
This means that great players do not have time to have great experience in those clubs, and all the players they have are either older players, or younger ones that do not have experience in the last games.