The club investment on the squad should always be based on the plan of the manager. It would be a waste of signings if the club sign players who don't suit the tactical plan of the manager. The problem at Chelsea is that they have lack of coordination between manager and the sporting director. Chelsea failed to put a right person in the position of football director.
If Chelsea decides to sell any of the big players they signed for a huge amount of money for another set of players, they will make a financial error, which will affect the finances of the club because they will lose a lot of money as no team will be willing to buy them with the exact amount Chelsea spent on them because the players respective quality and individual ability have dropped and same as their market price.
Their football directors did not know their jobs, they are after fancy players and big names, not after a good manager who will have a great future at Chelsea, That is why all the managers they signed since the new ownership are not staying for long because they have not given them enough time to build a team.
Selling players to buy another players will make Chelsea starting another rebuild. It will waste more times for the young talents to adapt and develop again. Buying players after selling players with a low net spent will be fine as it won't cause financial error that lead to breaking the FFP rule. It just will make the new manager to put more effort to adjust the players into a new dynamic.
Chelsea just need to help the new manager with enough data analysis to prepare a good plan for next season. This stressing point lies on the right football director to execute their recruitments policy. Signing hot names on transfer market should not be their priority anymore, they just need to bring in the right profile of players to play in a certain tactical plan prepared by the manager.