To be fair to Fergie, he left a team which, while ageing, had won 2 of the previous 3 league titles and only missed out on a third on goal difference. The squad needed a few tweaks and then would have been more than capable of challenging that season.
In hindsight, Moyes was nowhere near ready to take over. However, he was the master of his own downfall by trying to impose his own ideas on a team which just needed tweaking. As has been mentioned many times, getting rid of Sir Alex's coaching staff was a terrible mistake. We were already replacing a manager who had been in charge for 26 years as well as an experienced chief executive in David Gill. What we didn't need was a complete overhaul of the backroom staff.
I think Fergie's choice was probably influenced, ironically, by stability. He probably expected Moyes to provide a smooth transition from Fergie's last team to Moyes' own, an expectation which was quickly dashed. Van Gaal or Mourinho would have ripped that team apart as we saw first hand in later years. You could argue that they would have had the experience and authority to handle a squad of champions, but I have a feeling they'd have wanted to do it their 'own' way, particularly Van Gaal.