Let's not be simplistic here. I completely agree that information is only as reliable as the weakest link in the information chain, but that does not imply that the weakest link is necessarily the most recent one. The fact that I read what I claim in the "Da Vinci Code" is immediately verifiable.
Of course, but the most recent is the only one we have any control over. When determining the reliability of a source, we should certainly consider the reliability of the sources that source used, but we can't control what those sources were.