Xabi Alonso made an excellent decision to continue with the team next season. Leverkusen will play in the next Champions League, they will need a coach like Xabi Alonso because he has played in major competitions such as the Champions League. I can't wait to see the club compete with the clubs from other leagues, as we have seen how they competed with Bayern Munich for the Bundesliga title race this season. You're right, they need to add more players next season, so they shouldn't be eliminated so easily, because they will face difficult challenges in the next Champions League if they meet a major team like Real Madrid or Manchester City, we don't know if their current players will have strength to play this way next season.
I have no doubts on the performance of Bayern Leverkusen even when they participate in the champions league next season. They might not make it to the finals of the competition but I’m sure they will perform well and make it as far as the knockout stage of the competition. Even if they don’t have new players that will join them by next season, I still don’t have any doubts with them, they will still do well. Adding more players will be a bonus to them because of how competitive the champions league competition is as compared to the Bundesliga. Bayern Leverkusen was able to compete and outperform Bayern Munich in the Bundesliga, so they should be able to do as much as Bayern Munich is also doing in the champions league now.