Moyes and Mourinho are managers who aren't focused on implementing and coaching specific systems and patterns like Ten Hag and Van Gaal are. Van Gaal is obviously long removed from normal club football, and his vision didn't really match up which happens. But besides - they are all managers who weren't of the required level. Ten Hag just might be. Moyes is a mid table manager, not a top club manager. Mourinho is no longer a top club manager, thats why he's found his level now at Roma and the Conference League. Van Gaal is a national team manager, he's not suited to club management anymore (don't forget he kind of collapsed with Bayern before he was sacked). Ole was a nobody.
Ten hag is the first manager who is on the up, who has made his name because of his coaching, his progressive style of play, and has shown he's able to build a system that gets a group to play as more than the individuals, similar to Pep or Klopp. It's an extremely different appointment to anything we've done post Sir Alex. Pochettino would've been more of the same. This is us trying to build through just being a very well coached side with a modern style of play. He'll get his signings eventually, of course it's important, but the most important is his actual coaching team and the work he does with the team. Knowing how to implement his system and adapting it to be good enough to be a top team. The previous managers implemented what they wanted, they just weren't good enough.