Sebastian Haller is a really good signing for Dortmund now. Maybe it is very difficult for him to replace Erling Haaland but Dortmund wouldn't be in such expectation either. They must have just wanted to sign a trustworthy striker to be effective in attacking as always. They have made some moves for their defense too but I hope it can be enough for them. Because it is their weakest point to improve right now. Their weakness in their defense caused them to be out of the title race early.
It's true that their weakest point is in the defensive line, but for now after Haaland's departure I see there will be a new weakness created, namely in their attack line. I don't mean to doubt their striker next season, but I haven't seen his contribution at Borussia Dortmund. Maybe all my doubts changed when I saw him playing well for Dortmund.