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.
Also an excellent question. If you hold an anti-credit position, there is a "floor" of how low the price can go before the system forces it to close. This would depending on how much of your own bitcoin reserves are backing the position. The person who has this option open would be able to adjust this limit depending on how much risk they're willing to tolerate.
Remember that no one can just take a bitcoin loan for free, they're also guaranteeing that they'll pay back an appropriate amount of credit into the system (and hoping that the price goes up so it takes less bitcoin)
If I have 1 bitcoin of my own, and I accept 1 of yours for $400 worth of anti-credit, the price can only fall to $200 at the lowest before my margin is up, and I am automatically forced to sell back both 2 BTC for $200 each. Any lower than this, and the bitcoin I hold in my option won't be enough to cover the credit I've promised.