Baseball - Alan Trammell - I really don't see there being any chance of him being replaced.
Football - Here's a shocker for you all... Chris Spielman.
Very little chance of being replaced.
Basketball - Bill Laimbeer. Could definitely be replaced if and when I fall in love with basketball again. Used to be Joe Dumars, and before that was Ricky Mahorn.
Hockey - Steve Yzerman. Will never be replaced. Ever.