I don't think Xabi Alonso's proposed departure from Bayer Leverkusen at the end of the season is certain as the young Spanish manager is yet to publicly notify everyone about his next move
If Xabi Alonso leaves Bayer Leverkusen, then this will definitely be Bayer Leverkusen's final journey to become champions in the Bundesliga because the chance of winning will not happen a second time with a different coach. Xabi Alonso now knows the strengths and weaknesses of every player that Leverkusen has, so this coach is able to provide the right position for his players to bring victory. It is not certain that a new coach will be able to do what Xabi Alonso did.
If I'm not mistaken, Xabi Alonso and Bayer Leverkusen have a very good agreement where the club will give Xabi Alonso the opportunity to leave Bayer Leverkusen if there is a better offer from a big club, so the opportunity for Xabi to leave Bayer is wide open, and regarding the fate of this club of course will be uncertain because the important factor in how this club can turn into the best club in the Bundesliga at the moment is Xabi Alonso, he succeeded in building a solid team in just two seasons and he did it without the club having to spend hundreds of millions of euros in the transfer market as many clubs do if they want to make changes.
If Xabi Alonso leaves Bayer Leverkusen at the start of next season I think the club will understand it even though remaining at Bayer would be a pleasant thing for the club because he can prove that what Bayer is experiencing is not luck but the result of hard work and the opportunity to defend the Bundesliga title is wide open if Xabi stay in this club.