It is not about the WC players Napoli and Milan have, its about squad building.
Milan and Napoli have teams built coherently, players fit each other and the coach.
Everyone knows that Spalletti plays a 4231, and Napoli's squad is perfect for a 4231. Remove Anguissa for Zakaria, or Fabian for Locatelli, or Osimhen for Vlahovic, and it will work and these players will shine. There is an environment that will allow players to come in and succeed in defined roles. This is also why Lozano had such a difficult time, he didn't fit Napoli tactically. Adaptation took a long time then.
it's the same at Milan and Pioli has a couple of years head start of coaching this team.
Is our best setup a 433 or a 352? Or a 442, 4231, 4312... who knows. None of the actually fit because we have players that don't fit. We have 4 CFs, 1 LW, 2 RW, and until recently 3 midfielders that play the same role (Locatelli, Rabiot, Bentancur), etc.
Its the same problem Roma has. They switch formations every month. Mourinho has played variations of the 4231 his whole career yet at Roma he is trying all sorts of weird stuff because his players dont fit.
A player like Rabiot, Arthur, Kulusevski, Bentancur, Morata can fit a well functioning team and succeed. But they can't all be there together and play at the level we want.