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.
No, it's not a great point. If you dislike the motives behind Ripple, I'd understand, but people seem to be upset with a lot of things that aren't really big issues. A promise of being paid one dollar sure is not the same as one dollar in your wallet, but this is just the same in real life. In real life, you "solve" this by dealing with banks. In Ripple, you will probably deal with gateways having a role very similar to banks. It's a bit of stretch imagining Ripple gateways becoming as large and trustworthy as banks (well, if you really consider banks that trustworthy), but the concepts are in essence the same.
The problem is a line of trust does not work in practice. A trusts B. B trusts C. C trusts D. D trusts E. E trusts F. F defaults.
You ask B, and B tries to contact C. After a few days, B finally got in touch with C, and C looks for D who moved to France a few months ago.