That is not breaking the peg, that's breaking STEEM. the SBD would still give you $1 worth of steem which realistically i dont see ever going to zero. Sure it could go to near zero but someone some where will still give you something for it. even if it's 1 billionth of a cent then it just give you 1 trillion steem for it. But like i said the risk of it being worthless is why the SBD rewards you with a yield.