It is quite reasonable if people doubt Leverkusen to be the candidate of the champion in the next season. Bayern Munich looks stronger, meanwhile Leverkusen was declining their performance. Bayern Munich has many star players and there is no big change in their squad. But Leverkusen lost some very crucial players (Florian Wirtz, Jeremie Frimpong, Jonathan Tah). Even if Ten Hag is a top coach, he can't optimize the chance of winning the title with the current squad. I think people don't underestimate Ten Hag, people just try to think realistically.
Even Bayern Munich will start the upcoming season hardly without Muller who left the club after the end of the FIFA Club World Cup and especially with the injury of Musiala. Leroy Sane left them also for free.
Kompany might have to change his strategy because he will have hard times finding a guy with the quality of Muller and especially Musiala contributions couldn't be replaced easily by his substitute.
Since Bayer Leverkusen isn't in its best shape with what happened in the transfer window then I think Borussia Dortmund might dominate or at least be a pain for both of them this season. They showed good quality since the end of the previous season.