That's simply not true .
The reason he coached lesser teams in the first place was because of his failures in the early part of the 2000's .
After doing a good job at Barca in the late 90's, then failed to qualify for the world cup with a star studded Dutch side against a Mick McCarthy coached team. His next job was a return to Barca, where he was fired within 6 months. His stock took a huge hit at the time, and that is a major reason why he chose to return to Ajax as a d.o.f.
It was only after falling out with Koeman that he returned to management with AZ. As for winning the Dutch league, so did Steve McClaren with FC Twente, and they arent some major force either.
The reason he didnt win a treble with Bayern was simply because he was tactically outwitted by a superior manager in Jose Mourinho. What Jose realized at the time, that a lot of premier league managers have also figured is that you can simply allow Van Gaal's teams to hog the ball without much impact, then hit them on the break.Diego Milito and co did that to great effect on that night in Madrid. Yet, Van Gaal never learnt his lesson or evolved to deal with that. Its the same thing reason why relatively less experienced managers in the EPL have found him easy to deal with.
Van Gaal biggest weakness is a failure to evolve with the times. Its something a truly special manager like Sir Alex did very well, best shown in his period of success post 2005. That's how a manager becomes outdated.