This will probably sound like sour grapes but I think he also got a massive slice of good fortune with that youth team producing so many brilliant/world class players all at the same time.
For Beckham, Scholes, Giggs, Neville x 2, Butt to all come through the ranks at the same time is unbelievable and I don't believe that it was just down to Fergusons management. He was fortunate to get a bunch of brilliant professionals all at the same time - they had the perfect mentality to match their ability. This happened just at the moment they were starting to win the title again - hence they have no need to look for moves elsewhere.
It clicked at the absolute optimum time in Man Utd's history and Ferguson's career there.
As I said, I don't want to sound bitter and detract from how he developed and managed these players from their youth, but if fortune had not put them in the same youth side, who knows how the last 15 years might have panned out.