What happens if you hold 400$ worth of anti-credit and the price falls to 1$?  You would owe 400 bitcoins to close that anti-credit.

And on the flipside, I'm owed 400$ worth of bitcoins and I want to use my credit to purchase 400 bitcoins. Where do they come from?

And the problem exponentially increases as the price nears 0$.

BTW this feels like an options market, where obligations are paid with the equivalent USD amount in BTC.