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.
I think it is still quite early to conclude that a team's performance is in a worse direction because the team you mentioned could make a breakthrough again next season, although I am also not sure that Borussia Dortmund will reach the Champions League final again. However, we also have to understand that it is not impossible for Borussia Dortmund to repeat this as long as they have the ability and a much higher level of luck next season. Because last season, few people predicted that Borussia Dortmund would reach the Champions League final, but in reality they could get there.