I agree with you that Borussia Dortmund may not be able to repeat there performance they had on this season to this upcoming season because just like you said they defeated PSG and Atletico Madrid but with all the amendments and strategies those clubs has put in place there is no way Borussia Dortmund will beat them and besides it was even fortunate for them to have win them because normally PSG is more in a good shape than them so wining then was actually a narrow escape, however one of the things I even noticed about Borussia Dortmund performance is that there performance is always seasonal because at a moment they will be very inform and the next season they will start performing poorly.
To some extent, this season turned out to be unique for Borussia Dortmund, but only due to the fact that they were able to reach the Champions League final, without this, their season would have been very ordinary, and their main success would have been just reaching the Champions League zone in the Bundesliga. Yes, they were very lucky in the match with Barcelona, when Barcelona received a red card in the first half, or with PSG when the ball stubbornly did not want to fly into the net, but luck also plays an important role.
If we talk about Dortmund's luck in the Champions League last season, it felt really good. The reason is that they are quite underestimated and even getting through the group phase is very difficult. Because there are so many promising teams, but it looks like Dortmund can prove it with tireless achievements until the final match against Real Madrid.
I think all teams have the same opportunity as long as they don't get afraid or give up first with the existing situation. Maybe Dortmund will also find it difficult to repeat the same success as last season but they will definitely come back even stronger.