From a midfield tactical standpoint. It's a 2 vs 3 thing, the struggle of adjusting to knowing when to go and not go forward from a side midfielder in a 3 to that of being central in a 2.
Kante is WC destroyer and b2b player in the Gattuso mould but what Chelsea/Sarri need is more Essien then Kante. Now Kante has had all of his success playing in a 2 man midfield, whether it was with Drinkwater, Matic or Pogba. But now he is being asked to do more than ever before and that includes scoring goals.
The real issue for me is Kovacic, who is suppose to be the most attacking of the midfield 3 and provide goals and assists like Hamsik and Zielinski, but that isn't really his game. So they basically relying on the 3 forward players to score all the goals, which may work in Serie A, but not in the PL or in Europe. And when you play a midfield 3, one of the midfielders has to be the additional goal threat along with the forwards, but Jorginho, Kante and Kovacic just aren't that, they all prefer to play deeper. Kante doesn't need the ball but the other 2 need it to be effective. That's why Barkley has looked better in the midfield with Kante and Jorginho, while being technically inferior to Kovacic, he fits better as he is far more direct and wants to get into the box.
TL:DR
Jorginho isn't getting dropped and Sarri isn't going to move from a 3 man midfield. He was 4312 at Empoli and the Napoli players told him they preferred to go to 433, but the midfield shape didnt change
Wouldn't mind seeing a midfield 3 of Kovacic Kante Barkley.