I don't think that Leverkusen will end up like Napoli last season in the next coming season. The reason why I said this is that, there was a problem that made Napoli performance to drop and they started struggling on winning matches. Which is the absence of Spalletti, who left the club for Italy national team, and different coach took over who found it difficult to understand the players.
Xabi Alonso is still the coach of Leverkusen and nothing changes, which means that he can strengthen the club's performance better than last season because he knows that he has a big challenge ahead of him. Playing in the Champions League is quite big for Alonso. He might not be able to maintain his unbeaten record but he will do better next season.
In fact, Napoli fell so deep that this result would be difficult to repeat even if Xabi left. If I remember correctly, Napoli became the worst champion to defend their title among the top leagues in history. 10th place after the title is an incredible drop.
As for the Europa League, it has finally turned into an independent tournament and not a septic tank for loser clubs from the Champions League.