A default is not possible. The loans are fully secured by Ethereum. If collateral dips below an acceptable threshold, the collateral is liquidated and the debt is payed back to the system in full
> If you sell the Ether, you no longer have the Ether. If while you're holding the loan the price of Ether goes up, you benefit from that. Of course, if the price of Ether goes down, you're at risk of having your loan liquidated, but that's a requirement imposed by the system to maintain the Dai peg.