Mazzari is a pretty impressive coach. If There's anyone holding Napoli together other than Cavani, it's him. If he leaves(I don't think he will), Napoli will not finish in the top 3 next year. However, there is something worth considering when it comes to Cavani.
Assuming he leaves for a ridiculous amount that ADL is asking for, Napoli could potentially build a very strong team. True, they'd be losing their best player, but they could use that money to bring in 3 or even 4 excellent players. In any case, this will depend on whether or not Napoli want to be a competitive team, I mean, they could always pull an udinese and just become a selling club.
It will be an interesting summer. Serie A is at a very critical point. As Ive mentioned before and many of you agree with me, the departure of Cavani and others like him this summer from Serie A could be the final nail in the coffin. There is very little holding the credibilty of the league, Juventus being mostly it, and if other teams continue liquidating their assets then Serie A will reach a point of no return, at least for a long time.