The business strategy of not buying superstars but creating superstars originated from the financial crisis that made Dortmund miserable in 2004. Indeed, for many years, the team from Germany still exists and is a force in world football. The team's philosophy of selling talented players also makes it weaker, but they have earned a lot of money to maintain the team until now.
Dortmund last won the Bundesliga championship in the 2011-2012 season. After that, Bayern Munich began a period of domination. Assuming Dortmund can keep their star. Dortmund can also compete with giants Bayern Munich.
I think failing to convince a player to stay is the main reason and not selling them. What if player puts a pressure and declines contract extensions? Then obviously it will be loss for Dortmund later on as the player will move out for free. I feel that Dortmund should find a good way to convince a plyer to stay as like this they are only waiting for Bayern to become inconsistent in the league and get the title for themselves. Dortmund are just not putting any effort to win the title like this .