Many dortmund stars player go one by one to other teams and even many of them can actually get a trophy at their new club, every season dortmund always say they will try to stop bayern munchen in the league but in fact they always fail to do so, actually so far dortmund continue to have great players like the names you mentioned, but for some reason they keep selling these players even though so far dortmund is not a team that has financial difficulties, and strangely enough they even often sell their star players to a team that is their eternal rival, namely bayern munchen.
Maybe often the problem often isn't that Dortmund lets the players go, but the players want to leave Dortmund. The players are not patient and want to move on to a more successful team as soon as they can. Like that Dortmund fails to establish a good team in the long term