Dortmund has a good team and they are not weak at all. Even after selling many young and talented players in the last years, still, Dortmund was always racing in the top side of the La Liga with Bayern Munich while they don't spend too much money like Bayern Munich.
Also, Dortmund is getting what they need, they are racing in one of the best leagues with one of the best teams while they are also making a good amount of money which is what they want,
For sure they want and wanted to make some money in the last years but they also wanted to get the german title.
And at some stage its anymore possible to hold very good young players when they will get some huge more money at other clubs.
If Dortmund would be tried to hold all this players they would be run out of money and maybe dosnt stay as good as they are right now financially.