You are right. The change of coach will definitely have a big impact on Leverkusen's performance. However, if Leverkusen management had not sold talented players in the squad , then perhaps their overall performance would have been somewhat stable. However, Leverkusen is not a rich club. So they will have to rely on the money received from player sales to run the club well.
It is not possible to predict how dortmund , Frankfurt and leipzig will perform next season. Last season we saw that Frankfurt was able to perform quite well. The team was in third place in the points table. On the other hand , leipzig could not qualify for any Champions League or Europa League platform. Their only target will be to do well in the Bundesliga. This team will also fight hard to stay in the top four of the table.
Change head coach unfortunately will change the games of Bayern Leverkusen for next season regarding tactician under Xabi Alonso era will difference tactician from Erik ten Hag. Bayern Leverkusen have good side after defending several main players last season and only sell few players only although Erik ten Hag must find right replacement for several position leaving by Jonathan Tah, Florian Wirtz and Jeremie Frimpong.
Bit difficult for Erik ten Hag how three players above become important part for Leverkusen under Alonso era and so far Leverkusen not announce about new players for filling that position. Another bad side of Bundesliga easily selling their main players to rival team such as did by Dortmund, Bayern Leverkusen when selling their player to Bayern Munich.