Losing Frimpong and Wirtz was a major part of Leverkusen with Xabi. Look at the stats of how they played. Frimpong starts a lot as a starter and it's the same with Wirtz. I think Ten Hag will have big expectations and pressure because Xabi has left behind good stats. Ten Hag now just needs to work and make Leverkusen better. The German league is also not as tough as the EPL I think Ten Hag has a great chance to succeed with Leverkusen. In addition to these two, there are 3 more players being sold including Tah, Gustavo Puerta, and Odilon Kossounou.
Xabi was the reason why Leverkusen could compete with Bayern for the title. Losing him to Madrid gave no clarity about club's future. So Wirtz and Frimpong follows him. Leverkusen is officially back as a mid table team by losing those guys. Bundesliga seems to be a farmer league again by Bayern.
Bayer Leverkusen sold their key players; they also left Xabi for Real Madrid, and as you said, it’s a pure fact, Xavi was the one who managed this team to be able to compete for the Bundesliga title. Now they have changed their 2 key players and also their manager. Things will totally be different from next season, and nobody knows if it will be for good or the other way.
I think by next season everything will look new for this club. I just hope Bayer Leverkusen won’t mess up things next season because their 2 key players are gone and the manager is new, which means their new manager, Ten Hag, will introduce a new strategy. They will get 2 players to replace the players. Things will definitely be different.