Because Bayer Leverkusen managed to go undefeated last season and so, if the Bayer Leverkusen team still has the same manager and squad, then it is very possible that Bayer Leverkusen will still be able to get good results next season. Unless,if last season Bayer Leverkusen only had the luck to reach the final, then it is very possible that nextseason Bayer Leverkusen will experience a decline in performance. But indeed,any result is ofcourse still possible, but I personally am still very confident that Bayer Leverkusen still has the ability and a very big chance to get good results next season. Although indeed, having an unbeaten record is certainly difficult to repeat, but if only to get good results, I'm sure Xabi Alonso will still be able to do his best for Bayer Leverkusen next season.
If we are talking about Bundesliga, Leverkusen isn't impossible to make the undefeated record again. However, no guarantee that the same squad and the same coach will repeat the history. I assume the next season will be more competitive in Bundesliga, I'm not really sure that Leverkusen can be as strong as the previous season.
However, we must forget the history about Leverkusen in Europa league. They will be no longer in this competition for the next season, they will play in UCL.