I do think Napoli has a very limited bench, but I don't see those players progressing either. So they are never going to be good enough or Sarri just doesn't give them enough time to develop.
Most importantly though - Napoli are mentally limited. Today Sarri looked like a grotesque figure and Napoli were outplayed even more than they outplayed us in Turin. Yes, we effed one game and a very important one, but Napoli would do that every month at least.