I still respect Xabi Alonso no matter that they were unable to win Europa league because of his coaching skills which has made him to achieve a record that no coach have achieved throughout the history of Bundesliga. If it happens that Writz is leaving and some other players, he will get a replacement and he knows the player that he will be bringing over that he can do the job.
We might still see a new name from this present Leverkusen squad that will take over the scoring as we have seen that in the past with several clubs. I don't think that it will be a problem for Alonso if he loose some of his players that are after big clubs, he will still build up the team to be as strong as this. However, it will be much better and a relieve to him, if none of his men leaves.
I'm not really sure that next season Xabi Alonso can repeat the achievements they got this season because the habits of a team like this will make it much more difficult to maintain performance like the previous season. It is different from top teams like Bayern Munich because they have much better quality players even though this season they have experienced big failures in all competitions. Bayern Munich just needs to find a coach who has good qualities and their failure this season is much more due to the coach.
Having players come and go is a normal thing in football because there are many clubs that try to monitor the development of the players that Leverkusen has. Xabi Alonso will find it much more difficult to maintain consistency in the best form of play for his team next season because I am sure there are other teams who will try to make changes.
Now since the season is over, the teams are trying to improve the team and hire more and better players for their teams to have better performance in the next season. But, I think considering the performance of Xabi Alonso and his players during this season, the main goal for Leverkusen should be keeping the players they have and they should try to have a more stable team.
If they can have the same performance in the next season, they can be definitely the main team to race for the Bundesliga title and they can even have hope for the Champions League.