Your argument is just plain silly. Since the plaintiffs didn't have to "prove" the amount of damages due to unauthorized distribution, you unilaterally and rather brazenly set the figure at zero. You then use the figure used during a separate demonstration (by the defendants, I might add) of how easy it is to download music legally, along with the courts determination that unauthorized reproduction took place, to get to your 20,000-to-1 ratio. You do see that this is preposterous, don't you?
Essentially, your logical flaw is to apply a court case that has nothing to do with copyright to one that is purely about copyright. In the case you reference, it is in theory possible to determine the damages caused: just sum the quantities spent by each of the plaintiffs on doctors' fees and other medical expenses. It is impossible to do that in the context of unauthorized distribution of copyrighted material. This is why the statutes give the courts leeway in this domain -- as you have acknowledged elsewhere.
If we take your argument it its logical conclusion, the maximum damages for sharing a song should be $4, since one can never "prove" the quantity of damages resulting from unauthorized distribution. Do you really think that damages of this size are going to deter anyone from filesharing?