One player can make a big difference. Its also very deflating for players, fans, and the manager to know that they are just a feeder team for Bayern. Bayern don't have to beat them on the pitch, they just rip the heart out of their opponents in the transfer window.
i recommend looking at Dortmunds own buying policy
Reus: Gladbachs most important player when they bought him
Sokratis ? Werder Bremen's best defender
Immobile ? Seria A top goal scorer and the best of Torino by far.
Ramos ? more or less Herthas insurance policy last year and their best goal scorer... just look how they perform without him at the moment
Aubameyang: Number #2 goal scorer behind Imbrahimovic in the french league when they bought him
Mkhitaryan: set the all time goal record for the ukrainian league before getting bought by Dortmund
Kampl: absolute star player in the Salzburg squad who was key to their current success (109 games, 68 assists and 29 goals), voted player of the year in Austria 2014
Squads like Gladbach,Mainz, Freiburg,Frankfurt can complain about their players getting bought by the big teams ... but Dortmund, Leverkusen, Wolfsburg ? They better keep their mouth shut
(ironically i would rate Schalke youth academy output the best at the moment ... Bayern isn't too bad with providing many young players to the bundesliga top flight either)