As you said Willian would very likely do great in Seria A as I also think it suits him. But he has been with us like 3 years and it's safe to say he won't improve. He has good games here and there, but I really don't remember an outstanding match where he performed and won us a game alone. Each time we go in a game I feel like that flank is always dead especially when Willian plays, Pedro has been overall good this season despite his form dipping in the last month.
We can't get Sanchez, I don't believe Mahrez is an upgrade, Dybala is a pipe dream, James is likely the only top class player available right now, good enough to dispatch both Willian and Pedro. When there is an opportunity to sign such a player you don't simply say no, let alone he is very marketable and even if he flops we can always sell him to China because as I said he is very marketable player, even more than Oscar.