Dortmund have just lost Hummels & look likely to lose Auba (and Miki this summer or the next), and despite some promising transfers, it remains to be seen whether they will be able to compensate. Next season will be defining for them, I imagine, with regards whether they remain one of Europe's best teams and so, a team able to attract the best players.
I don't see why the OP is being destroyed here. For what it's worth, i'm not in agreement with it - I feel Dortmund will remain 2nd (as usual) and that Tuchel is building for a [potentially] very good future, but it's still a legitimate thought. The only genuine top class player BVB have left right now is Reus. They have a lot of quality in Kagawa, Durm, Sokratis etc, but year by year they have deteriorated on paper & the league table flatters them because no one else is good enough to challenge the top 2. Hopefully Rode, Dembele etc do well this year - I would very much like to see them win another Bundesliga.
Important players Dortmund have lost:
16/17:
Hummels - 1 year left on contract
Gündogan - 1 year left on contract
15/16
none
14/15
Lewandowski - end of contract -> Aubameyang
13/14
Götze - release clause -> Mkhitaryan
12/13
Kagawa - 1 year left on contract -> Reus
11/12
Sahin - release clause -> Gündogan
I think the last key player Dortmund sold willingly was Mladen Petric in 08/09, some might also count Alexander Frei, but imho he was let go because he didn't fit into Klopp's philosophy.
What I'm trying to make clear is that Dortmund is in the driver's seat regarding Aubameyang, which either means the rumours are rubbish (wouldn't surprise me one bit, since English media have a vivid imagination and none of the trustworthy German sources has picked it up yet)
or Dortmund actually want to sell him because they have a huge offer in front of them and a good replacement lined up. So this shouldn't worry anyone, especially since I believe that Aubameyang profits greatly from the team behind him and doesn't belong in the absolute elite bracket.
The other thing I'm trying to show is that Dortmund isn't deteriorating. Aside from Lewandowski (and Aubameyang still is a very good player) they have replaced every one who left with someone who is at least just as good while also strengthening their depth with every season.
This is also evident by last season's league performance where they finished three points shy of their all-time record, after dropping 5 points in their final two matches when title race was completely over.
Or if you look at it from another angle:
Sahin was the heart of their game before he left, now he is a luxury option and I wouldn't even be surprised if he left. Kagawa was the one who put the class in their attack before he left, now he is in and out of the first team. Götze was carrying the team quite often, now there were reports that the leadership isn't sure whether or not they want him back at a discount.
edit:
as I was writing this post Dortmund's press secretary told DPA that this is news to them and they know nothing about that story [regarding Aubameyang].