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.
That is why Dortmund is struggling since they normally sell their main players to other clubs, if a team sells key players, it means they have enough players in the team, but Dortmund no longer has crucial players, so they usually lose leagues and other competitions. I am confident that if Haaland Bellingham remains at Dortmund, he will be a difficult opponent if they do not defeat Madrid in the Champions League last season. Because I noticed they performed well against Real Madrid in the UCL but did not finish. Dortmund needs to stop selling its important players unless they have another player who can perform similarly.
That is why Bayern Munich are usually the favourites of winning the league title because Dortmund don't have better players like bayern Munich. And if Dortmund does not strive to buy experienced players, they will struggle to win the title. Bayern Munich sells Lewandowski to Barcelona and signs another top player, Harry Kane, which is exactly what Dortmund needs to achieve. I am confident they will win a title.