I don't think that it needs to be a fire/rehire situation. Rather, you have person A who takes care of systems X, Y, and Z. You have person B who takes care of systems W, U, V. And, you have person C who takes care of systems R, S, and T. Every n months, declare "shffle time", and have person A take over W, V, T; person B takes R, T, and Z; and person C takes S, X, and Y.
This will allow for the de-crufting of all of the systems by the new individuals who take them over. Of course, during the "shuffle time," all new project development would have to be suspended for, at least, 8 weeks.