If Dortmund improve their defence, they will play for the championship both in the league and in Europe, but no matter how many transfers they make every year, the same lousy defence does not change every year, they cannot show the success they show in attacking players. Nothing changed in Bayern also. I don't know if Tuchel is trying to make Bayern an England team or not. Maybe there were some dribble passing when Roben and Ribery were there, but I have never seen such a dribble passing-based offence in Bayern. That's why they already have problems against closing teams that press, and their defence is not as fast and skilful as it used to be and they keep conceding goals.
Even the attack force of Bayern Munich is currently not in it's best shape and form.as it used to be, I at some point even doubt if they will be able to really defeat Dortmund because most recently their wins have been mostly against teams that are small and to be considered as not measuring up to their strength a.s such team are the team they have been getting landslide victory over and it's looking like they are back to winning ways.
Dortmund would have been a much better and tough team if they breed their players and still maintain them in the bundesliga and not to train tham and still end up putting them away by selling them off, Dortmund currently have got small issues around their defense but I don't think it's enough to cause them serious loss against Bayern Munich even if it happens that eventually Bayern Munich gets to win them as usual, it will actually be with a little margin it wouldn't be with the usual big margin difference they usually do get against very weak teams.
Dortmund isn't weak and even their defense error isn't such that Munich will be able to really penetrate easily with their current form so I'm very optimistic about this game ending in a fair results probably a draw with almost balanced and equal performance because winning this game isn't giving Bayern Munich any advantage rather they will still be at their position. so they literally have almost nothing at stake winning or loosing this game.
The thing is if Dortmund was actually interested in winning, they would actually be able to win a lot more. Well, I'm sure that the players want to win. There are no players who actually do not want to win. But when the management is not doing a good job, it is hard for any player to actually win. At the same time, the coach has to be a good one. You cannot expect to win against clubs like Bayern Munich and, this season, Leverkusen as well with the mentality that they have. Previously we thought they were a real opponent for Bayern Munich even though the head-to-head comparison is nowhere near. But now they are just another Bundesliga club trying to make a profit.