Every decision may have been carefully considered, and Leverkusen's management has made slightly more erroneous decisions as several players and coaches have left for other clubs. But in football, this is normal, as players and coaches come and go to fill the void each season. What's even more problematic is the lack of preparation for the current situation, which has made the team worse. A new coach is expected to bring change, although he won't be able to do as great a job as Xabi Alonso, but at least being able to compete in the top three would be much better.
After success with Xabi Alonso, Bayer Leverkusen lost him to Real Madrid but not only this, they sold many star players to other clubs which are good for their club financial balance sheet in this summer. In long term Bayer Leverkusen will have to rebuild their squad with many new players with no guarantee of success and how they fell into crisis before Xabi Alonso appearance in their club is a warning for them.
Hiring Ten Hag as a replacement of Xabi Alonso is quite surprising but sacking him so quickly like this is even more shocked. Bayer Leverkusen likely take a series of action that leads their club to a wildly chaotic season.
Several managers with even better reputations were also available for free, such as Xavi. I'm honestly also surprised by Leverkusen management appointing Ten Hag as Xabi Alonso replacement. Additionally, seeing them release most of their key players in one transfer window, I think Leverkusen management wasn't serious about maintaining their existence as one of the elite teams in the Bundesliga. All the fans worries seem to have come true, so far Leverkusen hasn't been able to secure a single victory.
Replacing a head coach who made history with the club is not easy for any club as well as any replacement. Bayer Leverkusen struggled with this task so did Ten Hag but now they both have to move onward to the future. Xabi Alonso already became a story of the past, so did Ten Hag, while Bundesliga is on going and the season is far till its ending. Bayer Leverkusen must be careful in coming months as they have risk of falling out of top four in Bundesliga if their head coach position is not assigned to a good coach.