Honestly, it was half Barcelona not playing great, half the league not being that important anymore. They finished second, and Barcelona-Madrid combo finished 1-2 plenty of times before, even though Barcelona had some problems at the start of the season, the end result is something expected from any year and wasn't a shock, they need to thank Xavi for this.
The only downside could be that just because he had a good comeback and got them finish second, they could wait a lot from him next year, and may even spend some money. In that situation they shouldn't really lose hope if they do badly for a while, it will get better in the long run.
This is necessary even I feel if Xavi is not there then maybe we will not see Barcelona in the Europa League or Champions League next season, considering that for the beginning of the season until the middle of the season they really are not like Barcelona and fortunately Xavi is here and made a breakthrough that resulted in quite rapid progress here.
But towards the end I don't know why Xavi wasn't as intense as usual because if we look at the last few games before the end of the season their form wasn't very good.
Xavi is definitely the main reason why they are doing so well. We need to realize that Xavi is a marvellous coach, and he knows how to play the tiki-taka system that Barcelona played years ago and do it better right now which would be basically just a gegenpress version of tiki-taka that they played 10 years ago.
This is why he is such a great manager, he sees the developments in the world of football and he could put that to work as well. I do not know why they would ever consider firing him, but if they ever do because of bad results, they would be making the biggest mistakes of their lives. Even if they get a bad result here and there, they should be still be patient about him.