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 will be difficult for Dortmund to find a very energetic player like Erling Haaland, Although Sebastien Haller did very well at Ajax last season, I don't think he will be able to replace Haaland at Dortmund. But, back to your statement that it looks like Dortmund are not looking for a replacement for Haaland, they signed Haller only as a starting line-up for the main players. Overall, I also hope Haller can make a good performance with Dortmund.
It would be unreasonable to expect anything like that from Sebastian Haller. If anything they are trying to make him one of the premier strikers. After that, they are going to sell him again like they always do for a substantial amount of money.
And I think they obviously had problems with the defensive lineup. However, they also could not score enough goals as their main striker Erling Haaland was injured for quite a long time in the last season. I hope that this time they don’t have any problems like that and they will be able to play attacking football.