Just look at Michael Jackson. He was millions in debt, and now his family is milking him for all he's worth.
Besides the family, the drug dealers profit, although they suddenly lose a good customer.
Only their estate/heirs. Dead celebrities are often more popular that living, washed-up celebs. And sale of their merchandise usually shoots up right after they die.
But usually, the reason people give them drugs is that they _want_ drugs, and the sycophants they surround themselves with don't have the guts to tell them no.