Man U were utter shit under Ron Atkinson and when Ferguson was appointed, they were third bottom in the old first division and they appointed him because of his past record which was him winning 3 league titles, 5 domestic cup competitions and the UEFA cup.
He was the third horse in a two horse race and he won trophies on a regular basis, compare that to the Johnstone's Paint Trophy winners medal that our manager can fall back on.
Stability didn't make him a good manager, he clearly was a good manager before joining Man U and that's why he was given time, he didn't become a good manager because he was given time.