There was very little Todd Boehly could have done to keep Andreas Christensen or Antonio Rudiger at the club last summer. There wasn't too much to be done in order to stop N'Golo Kante and Jorginho running theirs down with uncertainty either. Outside of that, the contract situations have been kept largely in check.
Reece James was convinced to sign a much improved and deserved long-term deal, Armando Broja committed himself to the club too, whilst the new signings are all tied down. Talk of a new deal for Kante is ongoing and Chelsea are actively looking to get that done with terms favouring them.
Mason Mount is a different kettle of fish. In some ways his circumstances mirror that of Edouard Mendy and the issue for the players is that neither is in good enough form to warrant much power. That's the unfortunate truth.