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.
I completely agree with you, Xavi gave his all to the club even at the times that the management always complained about financial difficulties, over many years now Barcelona has been down with financial problems, yet Xavi was able to manage to team and win trophies, we should remember that is not easy to improve the players with a team that is facing financial difficulties. Let's not compare Flick to Xavi at this point, despite that he good manager, let's wait and see if he will be able to maintain the club in its current situation. I heard a rumor about Xavi departing with the club because he made a choice not to include Lewandowski in his squad next season, he needs to make different changes to improve the team, and the president said there's no way he can manage the club without using Lewandowski and they decided to sacked him.
Furthermore, Flick will become more successful than Xavi if the management signs more players to strengthen their squad next season because that will be the best option to pressure Real Madrid for the title since they will continue to dominate the league after getting Mbappe.