Every club has their strategy of making money. If that club cannot win trophies to make money they can buy players at low price and sell them at higher price and make money. This was exactly the pattern that the former Arsenal coach Arsene Wenger was using to make money for Arsenal. Dortmund is presently using that method to also make money because they produced one of the best young footballers in the world and they sell them without replacement.
I also think Ajax is known for doing something. I don't know if they used to have replacement for their own but as for Dortmund I know they don't buy high-profile players but they produce high-profile players
If Dortmund thinks this is the best way for them to make money, then they need to train more players to be strong to keep some in the club that will help the club win trophies , then sell the remaining players to make money than to sell most of their important players.
if we look at the history of player sales carried out by Drotmund then we will see how dortmund make money and that's all by selling players, so far dortmund has always been profitable when doing that, they rarely lose, one of their big sales is when selling haaland to man city, it's a dortmund custom and we won't see them stop doing it.