I'm not trusting the leadership of Ten Hang as Leverkusen head coach to continue where Alonso stopped or even raise the bar, in terms of maintaining the team's form and the position they've raised themselves to in the Bundesliga league. You mention his success with the Ajax young team some, but he didn't prove his capacity in Manchester United, and besides his success with Ajax were not actually replicated at Man united.
Heard some rumors about Cesc Fàbregas possibly coming to Leverkusen to replace Alonso, but still has some linkage with going to coach Inter Milan.
There is confusion everywhere and the fact that Ten Hag left Manchester United with no good result makes it more difficult for people to believe he would be fit to replace Alonso at Leverkusen.
However, Ten Hag lack experience in some high level competitions, meaning his move to Leverkusen might seem to be very risky for a club with Champions League ambitions.