Borussia Dortmund is known to be selling their best players in almost all the summer transfer windows and that is why Bayern Munich will continue to dominate the German Bundesliga for more seasons to come.
Jadon Sancho was sold last two summer before Erling Haaland was sold to Manchester City last summer and now Jude Bellingham has left for Real Madrid. I think the club's transfer strategy isn't concerned by the success of the team rather they're more concerned by how much they make as gains when they sell these players. They might also be lucky raise another great player before the end of next season and still sell him at the end of the season
I think we should not forget where Borussia Dortmund is coming from. In the late 1990s and the early 2000s they were at the same level as Bayern Munich or maybe even ahead of them a few times. In order to do that they had to make very high debts though which almost led them into bankruptcy afterwards. Then Jürgen Klopp came and the club has risen again and they even made it into the Champions League final back in 2013. Still they are pretty far behind Bayern in terms of financials.