Man Utd would barely have any numbers left to use based on this as surely Giggs, Beckham, Scholes and Neville's shirts would be retired as well as perhaps Schmeichel's and a few others!
Always find it a bit strange when clubs 'retire' shirt numbers just because a previous incumbent was very successful when wearing it.
The only time I would agree with it is for reasons like Man City retiring Marc Vivien-Foe's shirt number.