So who did it?
Yes, if the ONLY bit of evidence there is/was some nebulous or non-conclusive element, let the person go free. And prosecute the bastards that used such "evidence". Why should the citizens be on the hook for crappy investigative work.
My only question is how did they get a conviction in the first place, knowing what we know now? In the case of Mumia, forget the DNA. There's no question he was the perp. The LACK of DNA doesn't mean that the killer wasn't there.