Looking back to years of glory for BVB squad, you’ll see that only 2 players are missing (Lewa and Goetze) and I don’t believe the rest forgot how to play football. In that case maybe there are few problems with the team, possibly natural decline (injuries, exhaustion) but they are still too young for such eventuality, so there must be something else which must be resolved first before signing new players. BVB has management in place which is supposed to control situation and know the reasons.
Obviously they don’t see anything or are unable to do whatever is needed. In this case management should be replaced. Buying players for millions has not helped until now so buying one player or even 2 more won’t solve the problem either.