Fergie was always planning a few years ahead of the rest, that's why no club could match them year on year. Best anyone could do was concentrate their efforts on building up their current squad but Ferguson was always planning further down the tracks. He had earned that luxury. It also took bravery to replace people like Beckham, Ince, Keane, etc.
Something happened though as he approached retirement which stood to jeopardise his legacy. Their newest challengers were their city rivals. He faced the prospect of retiring with UTD in 2nd place behind City and he couldn't countenance that. He ceased to plan ahead and instead of replacing Ferdinand, Scholes, Giggs and many others with younger models, he held them together and used funds to add RVP instead. It was short termism so that he could retire at the top.
It's remarkably similar to what happened Liverpool in the late 80's when they exhausted their squad fighting off Everton, Kenny chucked in the towel, he knew what was coming and left Souness to take the blame.