Dortmund has been like that for a long time, they will never care about a decline in performance, and because no other Bundeliga team has risen so they remain safe at the top of the standings (below Bayern) because there is no significant competition from other teams, so I think this season it will remain like that.
And regarding the competition in the UCL, Dortmund will join the most difficult group with Newcastle, PSG and AC Milan. I think with Dortmund's current performance it seems like there is no chance for them to qualify from this group.
I would say Leverkusen is coming up with some decent competition with Xabi Alonso at the helm, and Leipzig has the finances to try to test Dortmund as well. Those two clubs are the four horseman of the Bundesliga along with Dortmund and obviously Bayern.
I believe that if Dortmund doesn't get any better, and just keeps on selling players without getting any good players in return, eventually there will be a moment where they just get too worse, and I mean like terribly worse. Schalke wasn't this bad neither, that was a team that won the league, and played at the top places for many years, before they started to just be terrible and went down. So, just assume that it is not going to be fine forever, they need to improve.