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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.
No I don't think his value as a manager would drop unless Leverkusen crashes brutally hard and finishes the season out of the top 4 or far worse than that. Everyone knows that realistically speaking, after such a season a lot of big clubs will reach out to those players who are mainly responsible for that remarkable success. Alonso has proven his value already. He took over in Leverkusen when they were crashing hard, and now they are on their way up with players whose names most people have never heard about before.
But since he now decided to fulfill his contract, I believe this will turn into a very important signal for everyone in the team and the club. It is not as easy a decision to leave the club as it would be when Alonso decided to leave the club. I think that most of them will stay and try to let that project prosper further. They have just built the foundation this season.