Bayer Leverkusen letting Xabi Alonso join Real Madrid this season and also selling some of their best players to other clubs this season, I agree that Bayer Leverkusen won't be a problem for Bayern Munich next season, and Bayern Munich will continue their dominance in the Bundesliga league because Erik Ten Hag needs to build up Bayer Leverkusen's squad to the level where they can be consistent in winning their matches and challenge Bayern Munich for the Bundesliga title.
We are living in a world where media could easily influence clubs administrations to sack a coach after few months or a season just for not finishing in the top places. Eric Ten Hag made Manchester United better even if he wasn't successful to get them back in top. Look at them when he was no longer a coach, they ended close to the relegation zone.
In Bayer Leverkusen the situation will not be any better after losing key players and the coach, so don't expect much the upcoming season. If he could maintain them in any European competitions it would be great.