In England players get testimonials for 10 years (give or take) service, I cant think of many shirts that have been retired here, the only one i remember is Foe's no.23 at man City, though that isnt so much for his contribution over the years but more a remembrance as he died in tragic circumstamces.
Instead a great player can contribute to the magic of a shirt, at Utd it is the no.7 as that was worn by Bestie, Coppell, Robson, Beckham and now Ronaldo. When a new player gets the shirt it is a sign of faith from the club that they can emulate the greats before them, and a motivator to the player who has something to live up to
