If Chelsea are willing to gamble losing him for free that is their prerogative. He is their asset and can demand what they want regardless of how we feel about the valuation as fans.
You have a good point.
Mason Mount has chosen United as his next destination which means other clubs have left the field. If he performs at a maximum level he would have suitors and would be in his prime age-wise.
Age is a massive factor.
Kovacic will have no sell-on value once his contract ends. Mount, should have a decent sell-on value if United decide to sell to the extent we could get the full transfer fee back.
Unless Kane signs a new contract ASAP it's suicidal from a financial perspective not to sell Kane. It will obviously hurt less to fans selling him to Bayern.