I know it's a terribly unpopular opinion but I don't think Van Gaal was all bad.
Incredibly boring at times, yes. Loads of home draws that were meant to change under Mourinho (and they did for a short period), yes. Arguing with players (as if it hasn't happened since), yes. Asking players to study things that aren't all pictures and have some letters in them, yes. 'Philosophy' and 'process', yes. Ultra ego (sounds familiar), yes.
BUT
We did have some sort of football identity, system and coherence. He did make Smalling and Blind look incredibly solid partnership for a season, introduced Rashford and got the best out of Martial. Some fringe players were looking decent enough, too.
What we lacked the most was absolute top quality in attacking positions. I think his system demands the forwards to be of the highest order, otherwise every move is finished up the opposition half, rinse and repeat. Needed that something extra to make it unpredictable. Should've also played it more humble and listened to the fans more.
What I know is that I've never seen us playing at Anfield the way we did under him. Total, utter control, almost robot-like plus some wonderful football on display. Before someone calls it coincidence - it's not. Controlling the game because of the right system is not a coincidence. Winning by jammy goals despite being under the cosh for 90 minutes can be called lucky but it was anything but. He had this string of great games (City at home, too) where we looked, the only time since Fergie, like we were going to become a genuinely good team. Great quality and control.
It finished quickly for some reason, some changes in personnel were introduced and perhaps he himself messed it up.
I just think that with him it would've been easier to improve. The system was there, with the right additions it may have worked perhaps. Now it looks a total chaos with no quality whatsoever. I don't even know where to start with. What to improve? The foundations under him were more solid I think. Turning 0-0s into wins is easier (imo) than fixing the mess we see now.
I may be talking out of my bumhole so feel free to point that out, it's just opinion.