I pretty much said that in my post, but couldn't think of a good way to simply explain my thoughts further.
In the end, all currencies are only as good as their ability to get a large number of people to agree in their approximate value. Yes, the dollar is no longer directly equated to a certain amount of gold, but there's several billion people who agree to it's approximate value...or at least that it has some value. I would doubt that there are several billion people who even know what bitcoin is.