I see a lot of doubts about Alonso's success next season, but they kept the team together, didn't let any important players leave, and are also working on new transfers. And don’t forget that Bayer is a well-played team, and Bayern, who obviously could be their main rival for the championship, still have to adapt to a new coach, and maybe this period of adaptation will be enough for Alonso to try to achieve a starting advantage.
Most people who keeps doubting the success of Xabi Alonso next season are yet to consider wha he has achieved in his first year as the Bayer Leverkusen manager. One thing he did that is good is that despite playing an unbeaten run in the league none of his players are seriously injured and the team is yet to sell any of the star players which should have affected the performance of the team next season, during the transfer market I believe Xabi Alonso will want to add more good players to his team which will make them stronger.
For sure Bayern Munich will be their biggest rivals next season but just as you said the team experienced some poor performance last season as a result of poor management but now that they have a new manager who will be tasked on reviving the team, I doubt if the team will get back to their best performance so fast so I still think Xabi Alonso and his team Bayer Leverkusen still has the advantage to be ahead of Bayern Munich next season if they should maintain their good form.
Next season, Bayer Leverkusen will face a more difficult task because they'll be playing in the UEFA Champions League competition where they'll meet some of the best clubs in Europe so I don't think they'll be able to maintain their performance and consistency of last season. That been said, I strongly believe that they have all it takes to still win the league ahead of Bayern Munich