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

This is just the Domestic League so it's not a great thing when only two big teams always dominate this League but they shouldn't just focus and be proud when they can dominate the Bundesliga because they also have to fight for the Champions League.
When one of the two teams can compete well for the Champions League and manage to have a chance in the final then that's a very good development.
Actually, Dortmund and Bayern have pretty good players, it's just that they still can't compete with other big teams for a prestigious title like the Champions League.