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.
I have my doubts about Ten Hag in this regard. We may have seen the situation with Manchester United where despite having sufficient funds, Ten Hag did not make the necessary signings and did not enter the transfer market sufficiently. That is why we have seen them lose their dominance in the Premier League. Leverkusen is a strong team in the German league and their achievements are also quite good. But I have my doubts about how Ten Hag will do in the future. However, I think if Ten Hag makes the necessary signings, we can see Leverkusen improve.