Of course those were really hard times for Dortmund. But it seems like they have learned their lesson from that as we don't see such unnecessary transfers. They are only focused on reasonable transfers in which they can make a big profit. Sometimes things don't go exactly as they imagine as well of course.
For example Haaland could have been sold for a much higher amount of money considering that he is a very special striker. At least the same thing didn't happen with Bellingham. They need to be more careful with release clause cases while they are signing such talented players.
I think that Haaland would have never joined Borussia Dortmund a few years ago if they would have not agreed on this exit clause in his contract. I am pretty sure that Borussia Dortmund has tried to sign Haaland without such a clause but that was just not doable because even back then a lot of clubs were already interested in him.
We should not forget though that primarily Borussia Dortmund is a football club and as such they should try to achieve the biggest possible success. With their current transfer strategy they make a lot of profit if it works out but it also means that as soon as the young talents show that they are great players, they are gone again. This is not good for the title ambitions of Dortmund.