The first of many, Bayern already trying to break over Leverkusen by hiring one of their players, and they are hoping many more would be gone by the start of the season and be bought by other bigger clubs. I feel like it's quite important to realize that Bayern has made this to Dortmund back in the day as well, that is how they got good and not get any good competition.
We have seen Bayern buy players from Dortmund, and end up getting Dortmund a little worse, buying players means that you do give them money, so they can get others, but they usually can't replace the player. Like Writz for example, you can pay 150 million to Leverkusen for him, you think they can spend all of that 150? Or even if they do, find someone who will fit that perfectly? Of course not.
About Dortmund who quite often sold their key players to Bayern Munich, but Dortmund more often always managed to produce other great players. Moreover, Dortmund do not waste too much money and that is proof that Dortmund also have good management even though they always fail to get trophies. So, if Bayer Leverkusen did the way Dortmund did, then I think the result would probably be the same as Dortmund did, because they still have good management also because they still have a great manager. But about Bayern Munich, although at this moment Bayern Munich may have managed to recruit some great players but because Bayern Munich's manager is Vincent Kompany, then I personally will still doubt the quality and chance of Bayern Munich next season.
It would be very disappointing if Leverkusen followed Dortmund transfer style, where they like to release their best players to competitors direct in the Bundesliga. I think it is very disappointing for the fans, Leverkusen would be better off releasing their players to clubs outside the Bundesliga. Bayern always has a way to weaken the competition, by recruiting mainstay players from other Bundesliga contestants, so that they can easily dominate the competition to become champions again next season.
Xabi must be able to retain his players amidst interest from several elite European clubs, his plans for next season could be damaged if most of his key players leave in this summer transfer window. Playing in the Champions League demands better squad depth, and it is not an easy job to defend the Bundesliga and DFB Pokal trophies. Bayern, Dortmund, Stuttgart and Leipzig will be the closest competitors in the race for the trophy. Regardless of Kompany chances in his first season as Bayern new manager, in essence the competition next season will be even more competitive, Xabi must convince his players not to leave.