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.
Dortmund does this more often than any other team, and this does not prevent them from staying afloat, last season is proof of this, when they were able to reach the Champions League final. Their work on the transfer market is very thoughtful, they not only sell their players but also buy very promising players with whom they can maintain good results, and at the end of the season they can sell for even more than they bought.
But such a strategy deprives them of the opportunity to become the champion of Germany, since they make too many changes in the team. If you look at the bookmakers' odds for the Champion of Germany, Dortmund's chances are estimated at 13.00, compared to Bayern's 1.52 and Bayer's 3.7. This makes it possible to understand what goals the teams set for themselves, Bayern wants to be the champion and is trying to preserve and only strengthen the team, and for Dortmund it is a business.