Look, there is no doubt that having their top players leaving for Bayern is not a good image for them, for the league, and to a certain extent for Bayern as well. We can try to spin it however we want, but a league is not competitive if the top players of the second team keep leaving for the first. And an uncompetitive league is not something people pay top money to watch.
On the other hand, they had better learn from the Lewandowski mistake and hopefully they can spend the money in a good way. Gundogan wasn't extremely missed when he was out with injury. Hummels will be missed a bit more but maybe Ginter has some ball playing qualities to supplant him but needs an arguably stronger defender next to him. They can and spend the rest of the money in arguably a better RB or LB. I think they can improve in both positions. Piszcek is not what he was and I have doubts about Durm and Schmelzer.
If they keep their front 6, add Dembele, they will have a strong team nonetheless. Dortmund needs about 10 straight years of CL revenues to really establish themselves and they are not that far behind.