If you want Xabi Alonso to prove himself for one more season in the Bundesliga, I think it will be difficult for Xabi Alonso to do that because Bayern Munich will not want any other clubs in the Bundesliga to beat them for the Bundesliga title next season. Secondly, Bayern Leverkusen will sell some key players, which will cause their performance to drop, and it will cause Xabi Alonso not to challenge for the Bundesliga title next season.
I agree with you that next season will not be this favourable for Bayern Leverkusen and I have already observed that on my previous post because Bayern Munich has been the most favoured club that has been winning the Bundesliga but after watching there performance this season and seeing how Bayern Leverkusen has taking advantage of the opportunity to dominate the league they will make sure it never happens again next season, so even if Bayern Leverkusen was not able to sell any of there players before next season but I doubt if there performance will still be good as this season, although I no that predicting what will happen next season will seem a bit difficult but with the way the season is going there is every possibility of Bayern Munich taking back the trophy next season.
That is the big difference between a club like Bayern Munich and Bayer Leverkusen. I do think that Bayer Leverkusen can and will play an important role in the next Bundesliga season, but will they be able to go beyond the 80 points mark again and maybe even target the 90 points like they are doing this season? As you said, if they can manage to keep their squad for the most part, they will still not be able to repeat that season. But there is already a ton of rumors circulating that Wirtz is a transfer target for Bayern Munich and several other players are being observes by big clubs. I guess they have to expect to lose 2 to 3 players.