It was going to happen. For 2 years now we've known Xavi's time as a Barcelona player was coming to an end, but it's still a sad thing when it happens. It's never fun when legendary players start to age and decline before your eyes. Always hurts.
It's good that Xavi is making the decision to move on, on his own terms. It's really hard for a top athlete to recognize when it's time to move on. Some never do and later regret it. Hopefully he leaves on the highest note with Barca winning multiple trophies.
Doubt this is a permanent goodbye. He'll be back in FC Barca in some capacity or other before long. Wish him all the success in the world in all his future endeavors, he deserves it. Best midfield I ever saw play.