Surely as a business, that is a football club, there's an argument to be made that monetary gains come from having the best and most famous players continue to play for the team to both build the global profile and help win prizes domestically and abroad.
Getting a one off payment for an asset surly isn't the better model for a team still on the way up.
Yes, most successful clubs usually have 2 or 3 players in the squad who are past their prime in monetary terms but are still very influential.