For the simple reason that $5k in debt isnt actually worth $5k (because of issues collecting and the fact that time = money) ripple will fail.
Great point. I can't wait till an exchange runs away and all the paid by opencoin ripple fanboys watch as their USD balance change from 500 to 0.
Might even start a bounty, "break Ripple's IOU system majorly and get 100 btc"?