I have never fired one of my players. I have released them, mutual agreement, shake hands and good luck in your new life, be it in a new team or retirement. Because I have respect for my players and I treat them with gratitude ^^
I don't know why the suggestion is needed. There is already a good system handling retirements.
The game doesn't retain players, managers do. If the player is deemed useful for someone, he'll be bought and stays in the game, if not he is retired.
There are heavily trained players who have hit their 30s now. They will lose skills but can still be very useful for some teams. It should remain the manager's choice if he wants older players or not. A bit of experience is sometimes unvaluable.
Good old players who are starting to lose skills is an important new component of the gameplay. It opens new ways of managing the team and it is creating a balance in the players "database"... and on the court too...
It shouldn't change, absolutely not.