His major health issues and the fact that it was the right time for him to go. Oh and Fergie knew the tide was turning and quit is Bollocks. Did he let that squad depreciate - yes. He genuinely felt he was passing on the baton to Moyes. Misguided as that was - he also admitted to sentimentality in not replacing Scholes and allowing the squad to age without addressing it. I think it was that he didn’t have the time and drive anymore to overhaul another team and create a new one. He’d done it a few times already and wanted to go out on top - which he did and handed it over to his protege. It didn’t work out but a revisionist version of this where Fergie cynically left united high and dry is nonsense. United were in decline but they went from champions to 7th? That’s on Moyes and the signings they made. His act of malice was poisoning Rooney’s rep before he left.
I very much look forward to the month outside the transfer window when Chelsea can't sign any players before they appeal and end up with no punishment whatsoever.