I don't think the board of borrusia Dortmund did made a mistake, it's been in their policy the trade of players be it their star or not, they make good profit of it and at same time are able to breed new set of players that still helps them stay a float such that they always maintain decent stance in the table most often than others does. What I see is happening at this time is the change in the era and breed of players such that what was applicable before may be fading away except if some adjustments are made to get it suiting into the new era least they get the team setting back with their performance and results.
Whether it was a mistake or not, it was a strategy for Dortmund to benefit financially.
Technically when talking about finances then they are clearly very good because a lot of players who Dortmund published have a very large value when used as business material which makes Dortmund's financial strength quite strong because of the profits they have. But when it comes to competition, it is clear that this is not really expected by some fans because of course they want to see their proud club get a trophy in a competition and Dortmund cannot do that because they have their own way of profit.
As long as there are no more demands from the fans then I think Dortmund will continue to do that but when the fans are still comfortable when they are seen as a big club even without any trophies that can be obtained then they will continue to do the same thing.