I don't think this is a big mistake for Napoli, because Spalletti also wanted to leave.
Leaving after winning that trophy is far better than stay with club a few more seasons and failing to get the title. Maybe Spalletti also thought of this, as we know, there are several coaches who take the same way. Zidane for example, he came out as coach of Madrid after get the title, as far as I remember Rijkaard did the same (CMIIW). Of course going after get the champion title will get a positive impression.
Yeah right. Deciding to leave when we were in our heyday was a wise thing. because people will continue to know us in good standing. well it looks like Spalleti certainly also thought about that. And he also has old age. so his decision to quit coaching was pretty good too. because he said he wanted to spend more time with his daughter. because maybe all this time he was too busy and didn't have much time to be with the family he loved.