Dortmund are the engineers of their own downfall. I can remember those years when Dortmund were the only team giving Bayern Munich tough competition each season but those glory days are slowly disappearing because Dortmund turned their priority to be a business oriented club and selling out every top talent they get for huge profits not minding the competition ahead of them. Leverkusen have now stepped up to become the contender for Bayern Munich. They are focused on competing and winning trophies.
Yes you are right Borussia Dortmund used to be great team but now they are not doing well because they care more about making money than winning games. They used to challenge Bayern Munich but now they sell their best players for a lot of money and do not think about how it will affect their team. This has made them weaker and now Bayer Leverkusen is strong competitor to Bayern Munich. Leverkusen wants to win trophies and be the best and that is why they are doing well. Dortmund needs to think about what is important again if they want to be top team. They made a mistake by focusing on money instead of winning and now they need to change.
It is undeniable that Dortmund is slowly starting to find it difficult to compete in the league because several teams so far have also begun to rise, so far we know that Dortmund is the toughest and biggest rival owned by Bayern Munich in the Bundesliga, but precisely last season when Bayern Munich slumped, dortmund also experienced the same thing so that Leverkusen, who had never previously been considered as a championship contender team was able to take advantage of the slump of Munich and Dortmund to run alone in the league, the poor performance currently owned by Dortmund is of course inseparable from the limited finances they have so far, so inevitably selling players is the main solution to keep them able to compete in the top tier even though they are not champions.
Dortmund has released a lot of their star players and they are no different from a factory that produces star players and then sells them to other teams so far, but I think it is very natural for them to do it to cover their expenses every season, even so I think no other team in the Bundesliga is able to do like what Dortmund does right now when they sell star players but they are able to give birth to a lot of new star players from the their squad and it continues to run every seasons.