> Precisely why brand drugs continue to make money over fist for pharmaceutical companies even after patents expire.

Generics may have the same active ingredient but (vastly) different pharmacokinetics - i.e. different absorption rates/retention in the body. For basic stuff such as painkillers that's one thing, but for more sensitive medication such as insulin, antidepressants or anything related to the cardiovascular system (heart rate, blood pressure and clotting) one has to be very careful when switching between brands.