Sorry I thought it's obvious enough so I didn't care to describe it.
Sure you can keep any players forever but we seldom do it, right? Because if you keep them long enough their skills will drop and play like amateurs. My idea to 'keep' them is by taking a snapshot of their skills at any point in their career, if others still haven't done this to them, i.e. no single player can be 'kept' by multiple BB managers.