I think Ancelotti did a deceptively good job getting them to 2nd last season. He was following on from 2 excellent managers (Benitez and Sarri) and hard to think they didn't peak really. The team now looks to be coming to the end of a cycle, lots of players heading into the late 20's and early 30's and not an awful lot of money available to replenish them. It's funny really, I get the same feeling looking at Tottenham (and I remember the same with Moyes at us at the end). I suspect having a manager of the quality of Ancelotti, with his ability to tactical manage games astutely masked a lot of wider issues.
In fairness to Ancelotti there is probably a lot of that in his career. He has followed top managers, at clubs with fantastic players but often when they were coming to the end of the cycle and he's kept the wheels ticking over as it were. He is very good at that, and I do wonder if he is the manager lots of people actually believe Mourinho to be? It's a big knee-jerk which I try to avoid, but I am not sure I would take a swap at this stage.
I think Napoli will hit a period of decline, and their owner may regret some of his antics.