What they are doing is not longer a mistake, it is a deliberate attitude and I think it's the way they plan to be carrying out their operations. Assuming it was a mistake, they would have realized the mistake after selling off a few players and see how great they become especially the goal machine Haaland. After Haaland they have built and sold players that are extremely doing well too. Assuming they are interested to build a squad ready to win trophy, they would have been convincing these players to stay with them and achieve positive results together. The fact still remains that they prefer and Prioritize business over building the team for competition.
But maybe as you understand that having a habit of selling some potential players that have been formed is much more profitable than focusing on the championship, and on the other hand I think that if Dortmund did not have a habit of selling the potential players they have then they would most likely be able to win consecutive trophies in the last few years by shifting Bayern Munich's position. Oh yes, I don't know yet whether Dortmund now has plans to sell its players again or not, but there is a possibility.
Borussia Dortmund has been doing this for years. They get the biggest criticism for this, but nothing changes. They see it as a business and think that selling players is more important than winning the championship. The criticism of football fans about Borussia Dortmund doesn't change the situation. They always sell their best players and try to find and train other players. They are actually very good at it.
Maybe this understanding will change and they will try to win the championship until the end and maintain their quality squad, but some clubs can't give up their habits easily.