Xavi's current position is no longer comfortable and I think his decision to resign at the end of the season is very appropriate because he was not supported by the Barcelona management to build this team well, but instead the team management set a high target and in fact some of the news about Xavi's last time in blame for Barcelona's decline in performance this season.
The arrival of another coach next season does not guarantee that Barcelona's performance will be able to improve and it could even be worse.
Barcelona management wants to support him but they have no money to sign top players to build the team. Xavi may feel uncomfortable because he can do the best with Barcelona team. I think he already did the best, but he may not satisfied with his own work. I can understand this because Xavi was a part of most successful Barcelona team. So, he doesn't want to see Barcelona as a weak team which can't compete for the title. Recently, I heard the rumor that Barcelona management will try to negotiate with him to stay in Barcelona. We don't how the final decision of Xavi, let's wait for until the end of the season.
I don't think Barcelona management blames Xavi for the failure of Barcelona. They must be aware that it is also caused by the crisis financial, so Xavi couldn't find the ideal players that he wants. He can't build a team as expected. In this way, it makes sense that Barcelona can't win any title.