Xabi leaving Bayer Leverkusen wasn't a mistake, because he was at the peak of his game and if he stayed, he would have just be a manager for a mediocre team. Yes he won the title with Leverkusen that is true, but if he stayed for another 5 years, you think he could repeat that? I think he couldn't, look at Bayern Munich right now, it's so clear that they are far greater team than back in that year.
Leverkusen would have been a team that fights for a UCL spot, and Xabi would be a manager who would try to make the team be in the top four every year, that's it, nothing more. With Real Madrid deal, he got to be manager of one of the best teams in the world, unless you are the manager for Bayern Munich, every manager in Bundesliga would prefer Real Madrid job.
No I disagree with you about Alonso and the club Leverkusen, if he had stayed at the club I don't think this poor form of the players would have happened because he knows these guys very well and the chemistry between him and the players wasn't like that of the Madrid players, it was one of the best. If he had stayed I believe that the players who left the club won't have made that move no matter what and Bayern would've done this season to be more competitive than they are enjoying now, is sad that Alonso made that disappointing move and got sacked by Madrid.
Now the champions league has teams who are fighting to finish in a more better position like the top 8 spot and if Leverkusen was led by Alonso it won't have been an easy one for some clubs but take a look at how they are trying to even get a win in the champions league and this might be their last time of playing in the champions league, they can't go far, in the Bundesliga table they are suppose to be in the top 3 but without Alonso they had to manage with what they have, I just wish for Alonso to come back to the club Bayer Leverkusen.