It's a big mistake from the management of Bayer Leverkusen to let Xabi Alonso leave the team after losing their key players that helped them to win the league, they may not have had what it takes to keep Xabi Alonso for many more seasons but allowing him to leave when they have lost so many players will definitely affect the team because the new manager will start a fresh to build the team and they may not be able to contest for the league title next season.
Xabi did decided to leave and not like they did wanted him to leave the team because in the two seasons they have had him, he has done so well for the team that they and now that they have lost a few good players, they need him the most because he understands the team better so he knows how best to coordinate the team to get them to winning games and also keep them in that position that they will still be considered a top team in the German Bundesliga.