Even when Xabi Alonso was with Bayer Leverkusen last season, their performance declined to the extent that if Bayern Munich dropped points, Bayer Leverkusen would not win their match even though they were playing against a weak team, which is one of the things that made Bayern Munich win the German Bundesliga title last season. Next season Bayer Leverkusen's performance will decline more because they will be without their best players, and they have a new coach, whom people doubt will succeed at Bayer Leverkusen.
Alonso’s departure will definitely be felt in terms of charisma and the confidence he instilled, but Ten Hag now has a chance to reshape Leverkusen into a different but still competitive team. The Bundesliga has often seen clubs rise when a coach maximizes limited resources, this doesn't mean Leverkusen should relent on new signings, If the take advantage of the transfer market and make some smart signing they will stand a better chance of competing for the title.