~snip~
I agree with you that the decision to leave or stay is a double-edged sword, on the other hand this is an opportunity to change fate and career to a high level, but it all depends on Xabi decision between leaving this zone or staying. and the possible difficulty of finding other opportunities. In my opinion, Xabi should stay longer at Leverkusen to prove that there he really has a big influence on the team's performance, not just luck. Xabi does have the qualities of a great coach. 1 more season with Leverkusen is enough and he can accept the offer.
This all depends on Xabi Alonso's decision whether to continue at the club he built or move to get a new challenge, but if he decides to go to Bayern Munich he still has the same challenge, just at a different level, not much different as now. he is at Leverkusen because he is in the same league, the difference is that maybe he will become a coach at a bigger club and this will be a challenge for Xabi Alonso in building good performance and not being easily beaten. If he is able to build Leverkusen which was originally an ordinary club. When he is at Bayern Munich he should not have to work hard to build a better squad like he built Leverkusen's performance, just use a style of play that suits the players that Bayern Munich players usually use so that he can improve his performance.
Xabi Alonso has been with Leverkusen in the last few seasons if he is at Leverkusen any longer then the opportunity to come to a big club doesn't come twice because Bayern Munich and Liverpool need a coach at the end of this season because their coach will leave so they need him now to be Next season's coach. We'll see what the coach will take at the end of the season, whether he wants to be in his comfort zone or out of his comfort zone.