This is very true, I do not think that it would be easy for him to repeat the same things again and again, there are situations where things are not going that hard. I believe that we are going to see them change some stuff next season and those changes (including selling good players like grimaldo) may make them worse. In that situation, the value of Xabi as a manager could drop, not a lot because he had this season, but he will not be considered as great as he is this season.
Look at Spaletti, he is managing Italy right now, and that's a good job no doubt but that dude managed to make Napoli win the title, I would have expected some major team to get him. Italian managers don't do well outside of Italy, so maybe that's why he didn't.
Running of a football club involves business profiting and the entertainment side, now since the value of most players in Bayern Leverkusen has increased because of the team's incredible performance this season ,that makes it a good profiting opportunity for the club to sell some of those players now their rates are on the high side and not when their rate dropped, therefore, I don't think they wouldn't want miss chances of profiting should big name club come with big offers for any of these performing players right now.