A few things about Xavi are clear and Flick proved them, if Flick lives up to other expectations and overcomes challenges ahead is a different topic.
When Flick came, people expected him to insist on a rigid setup which we didn't have the players for, apparently. Turns out it was Xavi reading his squad wrong and insisting on aspects of the game that didn't suit his squad. Even for Xavi's signings, it took a different perspective to unlock their potential. Be it with Lewa's role centred about him receiving high amount of chances and finishing, about which he often complained, making Raphinha work, or capitalising on Pedri's vision and natural verticality. Could go on here.
Xavi's overall attitude was toxic and plain wrong, often blame shifting and demoralising the squad in public. Made the audience believe their peak was lower than it was, not to mention how it influenced the squad.
Xavi was like a parent to a natural born athlete who he scolds and undermines because he's not great at, say, mathematics.
So it's obvious he was holding the squad back.