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.
If he received a good offers from other team, why he need to take a risk to prove himself for two seasons lol. It's either Fernando Carro increase Xabi Alonso's salary and give him privileges or Xabi will move to other team.
There was an interview by DAZN, it looks like Carro doesn't look want to give a special treat to Xabi, so I believe he will join other team.
I feel like it's quite important to realize that we are going to see Xabi causing a Spaletti situation at Leverkusen. If they keep him, and give him more money and more power, then he could keep this team better, but the owners/presidents of clubs do not like it when it is the manager that gets the praise and not them, they have spent the money after all, and they built the squad, they were the ones that got the manager too.
So just like Spaletti, we could see the management just let Xabi go, or basically make him feel unwanted, and he could leave. In that situation, just like how Napoli crumbled and became bad, Leverkusen will have a horrible season next year after this years title, and they will regret but they will not accept it's their fault.