Bellingham was Borussia Dortmund's biggest weapon. The same situation was true for Haaland last year. The team got a little worse after Haaland left. After Bellingham left, Dortmund got worse. Dortmund is a team that is adept at recruiting young and talented players. They may be bad this year, but they can make up for that deficiency soon. The observer team is very good.
I wouldn't at all compare Bellingham with Haaland and also not confuse the recent success as a goal scorer with Bellingham being a full mode striker. It could be and it will still take some time to see whether Bellingham can keep going and scoring. But the biggest weapon Dortmund had was Haaland for a while and Bellingham was very good, but he never as decisive as he has now been for Real Madrid. It doesn't happen very often that a player decides a few games in a row in the last few minutes.
For Dortmund Bellingham scored a goal around every 8 games. For Real Madrid he scored 4 goals in 3 games. To me this is most likely more of an outlier than the expected standard.
The fact that Borussia Dortmund is very good at recruiting talented young players who later goes on to become football superstars shouldn't be the reason why the club management should be their best players every summer and getting huge financial profits as rewards. That particular ideology of the club's heirarchy has really caused them the chances of winning major trophies for the club.
The sold Jadon Sancho to Manchester United after his impressive rise to stardom at the club and still allowed Erling Haaland to join Manchester City the following summer then sold Jude Bellingham to Real Madrid this summer. The club's continuous sale of their best players have aided their rivals in winning the German Bundesliga title for over the past 10 years