I don't really think this is true. Yeah you had a few flops like Sahin, Kagawa, Mkhitaryan and Götze (illness) but you also have players like Hummels, Lewa, Aubameyang and Gündogan (arguable) who performed well after leaving Dortmund.
Let's judge Pulisic and Dembele in a year or two.
But I can...