Van Gaal is one of the greatest coaches ever. You don't manage Ajax, Barcelona, Bayern and Manchester United for nothing! His system would fit the current squad better. The reason for the **** football was a general result of shocking recruitment. His philosophy was based upon Juego de Posicion (positional play). Some of the passing from the back was unreal. There were 3 big factors to his failure and the boring football:
1) The players weren't good enough. Whether Blind/Carrick was the pivot, you were over reliant on them in the first phase. Teams would just man mark the deep pivot and easily shadow your CMs as a result. Your central defenders other than Rojo had no verticality in their passing within the first phase. Jones/Smalling are poor on the ball. When Blind played there, strikers prevented him from passing vertically and redirected play to wide areas where it's easy for the defending team to force a turnover. The biggest hole was in central midfield. None of your midfield interiors were good enough. Herrera was the most ok out of the lot. None of them were comfortable with receiving deep. None of them were good ball carriers nor press resistant. They also lacked verticality. Pogba is the only United midfielder that fits the bill now. The reality is that your midfield wasn't good enough. Even now, take Pogba or Carrick out of the 3 and United are screwed. Herrera isn't good enough. He's a box to box destroyer who's tidy on the ball and happens to be good at counterpressing. Nothing in the final third and can't carry the ball.
2) Finding the right system was difficult given the lack of quality. The 4-3-3 is the best formation known to world football in terms of balance. Triangles can easily be created in every area of the pitch which is vital for beating shadows. It means that it's easy to overload any area of the pitch given the relative closeness of each individual within the field. It also allows all spaces to be covered making it hard for the opponent to overload. Given that it didn't work, Van Gaal tried the 3-4-3. Chelsea have tried this recently because Kanté isn't good enough to play as a lone pivot. The main advantage is that it allows for easy triangles to be made and it allows more spaces to be covered at the back. The 2 wide central defenders cover wide spaces, creating new angles while there's always a free man in the centre. United's buildup was even worse in the first phase because of Jones and Smalling. It did lead to the discovery of Valencia as wing back who would have shined in Van Gaal's system with more quality around him. Given the difficulty of adapting the squad to his system, he was constantly chopping and changing. The strong man oriented pressing scheme made it difficult for United to transition to attack. It wasn't organised in such a way to make it easy. It's difficult to do that and retain positional structure at the same times. Better sides can use individual quality to transition to attack, however United didn't have enough. This is why the ball was moved so slowly after winning the ball. The man oriented press led to a few bad results e.g. Losing 0-3 at Arsenal. Against teams which have virtually no positional structure (pretty much Arsenal) man oriented presses struggle with good off the ball movement,positions, fluidity and press resistance. This is why Arsene struggles vs Mourinho/Koeman. Zone oriented presses aren't as easily pulled apart by these factors. They perfectly retain shape. Van Gaal's complex system based on positional play requires many hours of coaching to implement. Very few managers can do it. Successful ones of note are Pep Guardiola, Marcelo Bielsa, Mourizio Sarri, Thomas Tuchel and Juergen Klopp. This is why Guardiola is the best in the world.
3) Poor recruitment was the fundamental reason to his failure. The players signed didn't necessarily fit the system. The wide fowards and creators (Di Maria, Martial etc) were perfect. A main problem were his strikers (shouldn't have sold Hernandez), his central midfielders and central defenders. Schweinstiger was a good snip and could rotate with Carrick in the pivot. Smalling and Jones aren't good enough with the ball and will never be. Bailly is young enough to be taught. United should have signed players like Umtiti/Mustafi, Hysaj, Raphael Guerreiro, Naby Keita, Granit Xhaka, Mahmoud Dahoud and Mateo Kovacivc. You missed out big.