Yeah that's right, there is no guarantee that a team will continue to play consistently every season we can take examples from several leagues, Napoli in Serie A and Leicester City in the Premier League well the same thing could happen to Leverkusen, I'm not underestimating Leverkusen but just making a comparison. If we look at some of the news, Leverkusen is indeed preparing and improving its squad for next season but we have to wait to confirm whether it will work well.
By the way, one of Leverkusen's strongest competitors such as Bayern Munich, is also preparing a squad especially in defense and so far Bayern Munich has brought in 3 players in the defense line with a budget of almost 40 Million, meaning they are aware of weaknesses on several lines.
A coach is certainly not difficult to build a very strong squad, especially if it does get full support from the team management, but the problem for every coach is to maintain the performance and also the consistency of the team in competing in the next season, from several leagues, of course we can see how some teams have to fall when the previous season they performed well, As you said, Leicester City and also Napoli can be an example of two teams that have a surprising appearance in one season but fall in the next seasons, if you look at Napoli and Leicester City, of course we find one similarity that we can probably say as the cause of their downturn, both teams fell after both sacked the coach.
While Leverkusen seem to be in a different situation between Napoli and Leicester City, Alonso is still staying at Leverkusen but not with some of their players leaving, I hope Leverkusen can keep their squad this season, so that way we can still see Leverkusen competing between Dortmund and Munich as Bundesliga title contenders next season.