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
Many people still don't understand the tactics used by some clubs like Dortmund and Arsenal at some point. It is like a farm to detect and to form talented players then sell them for a nice reward. Haaland was the best example, no one thought that Dortmund continued to win without him but they did win many times without having Haaland in the club.