Staying with Xabi Alonso increases Bayer Leverkusen's chances of retaining their title, especially if Xabi can get the same atmosphere as he got last season, getting the club's full trust to build a strong team because management intervention is often excessive which will create an atmosphere at the club it gets bad.
In my opinion, Xabi's task next season will be much tougher than last season's because there are two titles to defend and they will be playing in the Champions League, when there are bad results, sharp criticism will be directly addressed to him and that will increase the pressure on him as well as the Bayer players, no many clubs in the Bundesliga could win back-to-back titles if Bayer can do that which is very impressive, but I don't think they will return to their unbeaten record because that is something that is much more difficult for them to match.
I do agree that keeping Xabi was a MAJOR MAJOR play for getting success this season as well. However, we are going to be forced to think how they are going to do with the transfer period during the summer. They have some great players, and they may get some new players as well.
Either they could face issues with replacing any player they may sell, or even if we assume lets say they sell absolutely nobody, if they get someone then chemistry and fit may create some issue. I am sure that the yare not going to end the season with zero buys and zero sells, so that transfer period needs to be very careful, if they do any mistakes it would be a problem. I am guessing that it is going to take a while for people to see how it will change.