Moyes is a competent manager (as proven by his time at Everton). The problem was that the squad at United never gave him a chance. As an example, our defence last season was worse than Everton's under Moyes, despite being composed of superior players. That can only be because the individuals never believed in him.
How was ours composed of better players?
Baines is/was better than Evra.
Coleman is/was Rafael's equal.
And Distin and Jagielka have been a solid partnership for years - which is what it's largely about, more so than individual ability.
Meanwhile Rio was on the decline, none of the younger guys were near his peak level, and half of them struggled to stay fit for long enough to even form a partnership.
Plus you say "worse than Everton's under Moyes" as if they were bad. But they'd actually been solid under Moyes for a good few years.
And it doesn't follow that a defence of 'better' players will make a more solid defence. The balance of Everton's was better, and they seemed better drilled by Moyes than the defence Sir Alex left us.
I think if there was one area were Moyes was certain to improve this squad - it was in defence. I'd have had lots of faith in whoever he chose to bring in/sell in terms of defence. Strikers, on the other hand, I'd have been worried.
As much as Moyes had his faults, I agree that a lot of the problems last season came down to a lack of quality.
Assuming he had good money to spend, he should have strengthened our mid-table midfield - and I guess you could argue that he should have pressed hard for Thiago early on even if he didn't have much money, but now, with all these new players, you really see how mediocre so many of those players were.
And I agree that he was let down. I'm sure a few of them liked how they'd had things for years, didn't react well to his methods, looked at their medals and thought "Whatever happens, he'll be gone before I am" and choose to not even meet him halfway.