Haaland does have a huge impact on Dortmund's performance as he's scoring a large percentage of their goals and there is no one denying the ability of him in terms of scoring goals.
But I have to say that when Haaland was injured, Dortmund didn't have a huge lot of problems in the scoring department. Maybe they struggled in a few matches but they overall did okay without him.
To understand the importance of an individual player, you need a distance greater than a few matches, football is a team sport, so the individual contribution of even a star player is very difficult to evaluate. For example, when Ronaldo left Juventus, the team found itself in a deep crisis and this manifested itself very quickly, although formally there were two more very good strikers in the team - Dybala and Morata. Haaland is very important for Borussia and they will have to look for an adequate replacement for him if he leaves.
I think the same, the advantage of the German League teams is not like that of the Spanish League, because when Messi left Barcelona everything collapsed, in fact Barcelona has not been able to recover, while Dortmund has managed to get ahead with his entire team, almost without depending on Haaland, which is a plus for the team, and this should be recognized both to the players and to the technical director in general, although Haaland can go to any other team, I don't think Dortmund look so bad, Haaland's injury has helped him open the doors in teams like Real Madrid, MU, among others.
For Borussia it would be better for Haaland to stay, but I agree that they will do well without him as well. Messi's leave from Barcelona made a really big mess, but Borussia was already in a similar situation when Lewandowski left and she survived it without much problem. This time, the team will also be able to handle it, the only question is who could replace Haaland.
Borussia Dortmund has a system that works great. Of course, Haaland's absence is felt, but other players make up for it by making a lot of effort. In Messi's example, all plans in Barça were made according to Messi. Messi could have scored or scored alone. However, harmony between the team is very important in the German system. They are one of the countries that play the team game best. Borussia is one of the teams that does this best.