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
If they were able to retain the good players, Bayern could not have been dominating the league for years. It takes a consistent team to have great success, it's only financially good to sell a player who can make money without using it to another team.
On the one hand, they are doing something quite logical, they always sell their players when they are at their most valuable and make a lot of money. But their chances of becoming champions are dwindling

I wouldn't be surprised by the transfer of players who contributed a lot to Borussia Dortmund, I wouldn't even be surprised when I saw them lose all their players. lol
Borussia Dortmund puts profit above performance, so they have always done things against many people's wishes in order for them to keep their quality players. I really understand that in the world of football, business cannot be separated, but they also have to think about achievement. If they can't put achievement before profit, then at least they can equalize the two, meaning they have to get both things.
But until now we can see how they act, and that makes it difficult for them to compete with Bayern Munich.