I don't think steroids are to blame for Beane's loss of competitiveness. Sure, he had some high profile steroids suspects when they were competing (Giambi and Tejada), but it was the loss of Hudson, Mulder, and Zito that killed that team. He hasn't been able to build a competitive team since they left.
Plus, as usual, I'm going to point out that (A) we don't have any idea how much steroids help (but there is good reason to think it is less than is generally assumed), and (B) the decline in offense was small after drug testing began until they got serious about amphetamines. Then offense crashed.