Maybe they're comfortable in gaining financially from their player sales than winning trophies, cause if the trophies were important to them, then I don't see why they'll decide to sell players that are capable of winning titles for them, if you noticed their performance last season when they had Jude Bellingham in the team is not the same this season, they almost won the Bundesliga when he was present in the team, and he's now performing well at Madrid, I believe if he was still with them they would had been doing better.
What I feel is that it's become a tradition for them and it may continue for a long time, they'll still try as much as possible to qualify for the Champions League and sell some youngsters that are currently in the team, maybe Karim Adeyemi might be the next Youngster to be sold by Dortmund, but where it could start affecting them is when they fail to find a better replacement for the players they're selling.
Maybe Dortmund have to change their habits a little if they want to be involved in the title race, selling mature players will only make a profit for the owner but no profit for the club. Selling Jude Belligham is not the first time they have done this because previously they sold Haaland and several other young players. Club owners don't seem to care about the club's achievements when they take part in competitions because the most important thing they think about is profits.
If the club owner does not change his vision then I am sure it will be quite difficult for them to achieve success, both in the Bundesliga and in the Champions League. They don't buy finished players but instead create players to sell to other clubs, that's why when players leave it will be quite difficult for them to get ideal replacement players.