It is always difficult to maintain the same performance in different seasons, there are many teams that fail to meet those expectations. Just like you, I also doubt that Leverkusen can maintain their performance next season, especially after they qualify for the Champions League. Several other teams such as Leicester, Napoli and others have failed in different seasons even though they won the domestic title.
There is another team that I doubt too next season, they are Girona from La Liga. Girona and Leverkusen have been two surprising teams this season, but Leverkusen have been much more consistent and they are almost certain to win the Bundesliga title. I find it difficult to expect these two teams to be able to maintain their form next season, unless management succeeds in building a better squad next season.
That's true because Leverkusen doesn't have quality players like some clubs who consistently play every season and Leverkusen's chances of maintaining consistency and playing the same as this season are still quite questionable. Qualifying for the Champions League will make Leverkusen have to prepare themselves to be more prepared by relying on players like now. I doubt they can play consistently because the busy match schedule will make a number of players tired and could also end up with injuries.
So it will be increasingly difficult for Leverkusen's strength to match the match when facing crucial matches in the Bundesliga and Champions League. They are a surprise team that is playing well in their respective leagues and it is not certain that in the next season they will be able to maintain the same form of play. This concern occurs because they are not a team that has equal quality players so it will be a little more difficult to appear consistently in the next season.
When I heard that Xabi Alonso had decided to stay with Leverkusen, there was a high sense of optimism that they had for the next season, even though we were worried that Leverkusen performance would decline significantly from this season. A risky courage was taken by Xabi Alonso when he decided to stay together, apart from the reason of wanting to hone his skills to be better, in fact Xabi Alonso also saw the opportunity for them to achieve success again. For some people, his courage to refuse tempting offers from clubs like Bayern Munchen and Liverpool is questionable, but on the other hand, Xabi Alonso has shown a quite rare attitude towards all of us, not just limited to loyalty. It's about faith and mutual trust that together they can repeat a season as impressive as this one, so that they will be able to overcome any challenges.
Meanwhile, Xabi Alonso will definitely add depth to the squad to make it even better, if he succeeds in winning the treble winner then this plan will be easier to realize, because Leverkusen will get more bonuses. Xabi Alonso decision to stay could also influence the players decisions, I think most of their key players will choose to stay for at least one more season. By having better rotation options, Leverkusen can balance their performance in every competition they participate in, especially when playing in the Champions League and competing with other elite European teams.