They broke the bank on Maradona (when they didn't actually have the money) and also spent money re-building the team. It took them 3 years AFTER they signed Maradona (also signing de Napoli ,Ferrara, Carnevale, Giordano etc). They had finally built a team adequate of winning Serie A but also had a star man to take them to that level. Just like you get people in the Transfer thread saying we NEED that one guy to take us to the next level.
Championships are built on defences. They had one of the meanest defences in Italy. With Maradona's magic, they also had end product. Napoli's finances deteriorated (due to spending what they didnt have), had to sell players, Maradona was banned and then sold and then they ended up going into decline and getting relegated.
That has nothing to do with Maradona leaving them, this was just stupid ownership. All the other Giant Italian teams like Milan and Juventus were buying all the top players and even if Napoli had Maradona, probably wouldn't have accounted to much, obviously that's just hypothetical but thats mainly the reason why they hadn't done much since he left, poor financial decisions, relegations etc. since theyve come back into prominence in recent years, theyd probably have had more titles if it wasnt for Juventus' clear dominance.
Once again, I'm not dismissing Maradona's effect on the Napoli team. He was there at the point he was CLEARLY the best footballer in the world, all I am disagreeing with is the fact that he won the league single handedly.