Xavi has proven his capacity and he has given the title of La Liga and the Spanish Super Cup to Barcelona while the team is down with financial problems. I think it is not appropriate to conclude that Flick is better than Xavi. This season Xavi can also be said to have worked quite well, but he did not get full support because of some things that made internal problems even more tapered. Flick is also a great coach but able to prove that he will be better than Xavi and in my opinion all of that will not be easy.
Xavi did the best he could but it appeared to the management like it wasn't enough and so they had to relieve him of his job even after all he's been able to achieve with the team so far this season, he has even been able to manage the team through all they had to face this period and was still able to get to win some trophies along side, that alone gat me believing he's a good Coach and Barcelona would have given him some more time.
I don't know how well flick will be able to prove to the fact that he's a better replacement for xavi amidst all of the challenges the team has got to face at this time which actually isn't even looking like the team is getting pass it pretty soon, I don't know if flick is willing to work by the term as I saw on the local media that they are yet to confirm the deal between him and Barcelona, when xavi was still the coach, it was quite obvious how he was loved by the players probably because of his relationship with them, I don't know if flick will be able to maintain same.
The relationship of the players with the coach aswell has got a way of affecting their performance as clearly seen with ten hag at Manchester united so it's a very important factor that needs be considered by the team actually, because if the team will be able to regain dominance in the la Liga again ahead of Madrid, then they will have to make sure they do much more than that which they have done with xavi.