If Leverkusen extends the contract of Xabi Alonso they will not let any other teams including Liverpool or Bayern Munich even think about their coach to hire Xabi Alonso for the next season. But the fact about Xabi Alonso and Leverkusen is they are going to play in the Champions League for the next season and if they have Xabi Alonso, they will be a strong team in this tournament. However, I think Leverkusen will also need to hire some players and get improve to make them ready to play against stronger teams like Real Madrid.
Sadly we don't have Leverkusen in the Champions League in this season, but I'm waiting for it until the next season.
Apart from proving of course Alonso's decision to stay in Leverkusen is none other than because they will play in the Champions League next season, indeed Bayern and Liverpool will also play in the Champions League but indeed handling the team that has been built is certainly easier, than if he had to adapt to new players and squads in Liverpool and Munich later, Leverkusen's form so far in the league is quite good but still they need improvement when it comes to playing in the Champions League next season.
At the moment the squad owned by Leverkusen is indeed very ready to face the challenge in the Bundesliga, but instead they seem to struggle when playing in the Europa League like now and of course playing in the Champions League will have a higher level than the Bundesliga and Europa League so far, I think without the improvement of the team by recruiting some star players of course Leverkusen's journey in the league next season will be short and maybe when they Just facing another European team weaker than Madrid.