I'm not sure I agree with that. For one thing, "intelligent and sensible people" should have known that safety and above-average returns are usually mutually exclusive. The reliability of those returns should have been a tip-off that the whole thing was a scam. Secondly, one of the key items I read about early in this case was a comment by a CPA who said that anyone who could read a financial report would have been tipped off to the scam right away.
Hindsight is 20-20.
In fact some of the best financiers, such as Warren Buffett, have provided well-above-average returns consistently; just not as consistently as Bernie Madoff.
Even finance professionals were taken in by Madoff, and only a few figured out it must be a scam.
The old saying is “you can’t cheat an honest man”, but Bernie Madoff proved it wrong. He was a diabolically clever scammer.