If Bayer's fans are so naive that they expect a Bundesliga title this year, then they needn't worry - if Alonso leaves, it won't be until next summer. Bayern are showing dubious results so far, but still their chances are higher than those of other clubs, take a look at the bookmakers’ quotes:
Bayern Munich 1.24
Bayer Leverkusen 9.50
RB Leipzig 11.00
Borussia Dortmund 12.00
First of all I do not think that contracts matter for managers, if you pay the compensation you can handle it because there is no transfer fee for the manager. So, if any big club would like to get Xabi Alonso during the middle of the season, they would have to pay only just few million dollars, something small like 1 or maximum 2 million euro and they can get him.
It's more important to convince him and I am sure he is happy where he is right now and only would consider this if he is offered something huge. I believe that he will stay though, I do not see any team that would be better than what he has right now that could get him. Even if United or Chelsea fires their managers and want to hire Xabi, he knows that he shouldn't risk his career and go to a place like that where it would only hurt him.