In essence, Dortmund will always be willing to sell all their key players to clubs that have money and are willing to pay. So, even though Dortmund sold their key players to Bayern Munich, basically it was like a tradition to be done by Dortmund for a long time. But the great thing about Dortmund is, even though Dortmund always sells their key players every season, but every season Dortmund also always manages to be Bayern Munich's competitor at the top. Also, in the second point you said about "no team wants to compete with Bayern Munich". I don't think that's right because after all, every team is always trying to stop Bayern Munich's hegemony but yes, because Bayern Munich has good consistency it is certainly difficult for other teams to be able to compete consistently with Bayern Munich.
That's Dortmund business so far, developing players and selling them at high prices. Yeah although Dortmund sold important players, they can still stay close with Bayern Munich in terms of points, but still they couldn't take advantage of the situation because before they should be have won the title. I hope that in the next new season Dortmund can also be consistent again although they already sold important players again in this summer transfer window. You are right that other clubs every season try to stop Bayern Munich hegemony, but they difficult to be consistent. Now other clubs are improve the depth of their squad, which one of them is Leipzig and it is hoped that with the addition of new talented players, they can can stop Bayern Munich hegemony.