Whenever I hear the media and the government denouncing loudly in unison, I look for the possibility that the target is a scapegoat, and that the scandal goes deeper, touches better-established personalities.
This article by Wayne Madsen (scroll down to see it) offers a plausible hypothesis why Madoff's conviction is the end of the line, and prosecutors aren't lookig for co-conspirators. According to Madsen, Madoff's investors were not so dumb, and what Madoff was really collecting money for was a vast floating casino project in Manhattan, with new transit lines thrown in for extra $$$. The "Cornerstone Project" was to have absorbed Madoff's cash and rewarded him richly, before last September's crash put the project in jeopardy.
I won't try to summarize the intrigue that Madsen describes, aside from revealing the "cornerstone" of his charge: Judge Denny Chin, who has accepted Madoff's guilty plea and will be in charge of sentencing him, is a major investor in the Cornerstone project.