One thing we need to understand is that not all clubs will be able to keep their most important players at the same time, some clubs are definitely just going to be developing talents and then letting them leave, clubs like Benfica and Ajax can also be good player’s example again. This clubs are built the same and no matter how they do well in their league they will end up selling their players, Dortmund isn’t different from them, even if they win the Bundesliga today they will sell off their best player to their direct opponents self, I could remember the same issue with the Lewandowski then when he moved to Bayern Munich.
I also agree with you some clubs like Benfica Ajax and Dortmund are like training schools for young players. They help players to get better but then they sell them to bigger clubs. These kind of clubs make money by selling their best players instead of they do not take any season seriously and do not win any trophy. It is sad for fans to see their favorite players leave but that is how transfer market works. Even if Dortmund wins league they will probably still sell their best players to top clubs like Bayern Munich. This makes it hard for these clubs to be successful for long time and compete with best teams in Europe.