How do you know that the club hasn't worked hard to bring in a top drawer centre half and another midfielder? We've probably spent the best part of three years following Mats Hummels to no avail. If the player won't move then the player won't move.
As for the midfield, we've made strides towards improving it with good signings like Herrera and Blind. We're not completely there yet, that is true, but it shouldn't be forgotten that we didn't sign a central midfielder for over half a decade. And when Moyes had the chance to act on Ferguson's stupidity, he bought Fellaini and Mata, both of which did nothing whatsoever to improve the issue.
We were never going to completely rectify the midfield in one transfer window, realistically speaking. Christ, we've only just managed to get rid of Anderson. The midfield is still a work in progress, and it will be improved further come the summer, but people are being very disingenuous when they suggest that the midfield hasn't improved. No football team dominates possession the way we do without good midfielders.
As for Di Maria, he was an imperative purchase. Our wide men were and still are rather horrible and we had, and still have, a general inability to beat an opponent all across the team. Whether or not he is in great form right now is secondary to the fact we desperately needed a player like him at the time. That hasn't changed.