Don't be surprise they lose beyond just 2 to 3 players after the end of the season, Borussia Dortmund is a typical example of the level of a club like Bayer Leverkusen, for management will want to make profits off these players as bigger teams in Europe come for them with an irresistible price even Bayern Munich is sure to have a share in one or two these player when the transfer windows set in. And it's for this reason we see I'm consistency in the performance of these teams not been compared to that of Bayern Munich each passing season.
Rumors has it that Alonso rejected a call for a replacement of Klopp at Anfield Liverpool, may be if Alonso plans on staying in Leverkusen for the next season then he could persuade the club management and the players to still remain with him in the team to continue with the level achievement, otherwise then Bayern Munich will be recovering the top position all through as usual come next season.
There is a little difference between Leverkusen and Borussia Dortmund as the latter has to justify any decisions they make as they are listed on a stock exchange. I don't know how exactly that has an impact on the decisions the club makes, but I believe that Dortmund has less scope for taking risk and making errors. Leverkusen, however, can take some more risk I would assume, but they are in the same position eventually. If a club comes around and says they will pay 120 million for Wirtz, can a club like Leverkusen really resists? I don't think they can.