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 don't think that Dortmund will be bad this season even though at the start of this season Dortmund was not yet at the top of the standings after playing three matches in the Bundesliga. But Dortmund still has a lot of matches to prove that they are still strong enough in the Bundesliga, even though Dortmund might not be that good in the UCL, but if it's just in the Bundesliga, I think Dortmund still has to be taken into account by its opponents.
Borussia Dortmund has always sold important players, so it didn't really matter with Bellingham's transfer to Real Madrid and Dortmund will still be the top team in the Bundeliga, but it must be admitted that this season they won't be as strong as last season and also of course the competition at the top the Bundesliga standings with Bayern will not be as competitive as last season, Dortmund will still be Bayern's shadow.
But you also have to watch out for there are several Bundesliga teams that have started to develop much better this season and that is a bit of a threat to Dortmund at the top of the standings to fight for a ticket to the UCL.
I don't agree with you because a palyer like Juge Bellingham is important for any team and Dortmund surely spent too much for growing a palyer like him because he also had a good performance when he was playing on that team and now you can see this palyer is even shining in Real Madrid. But Drtomund can still be on top of table Bundesliga table with or without Bellingham in their team.