Another solution would be to introduce in Steem Bitshares UIA and have gateway USD listed, with a XXX.USD / SBD book and XXX.BTC / SBD book. People who would wish a avoid the risk of having their SBD transformed in Steem in case of black swan would convert to XXX.USD or XXX.BTC. It also makes it easier to withdraw: they can directly withdraw the XXX.USD to XXX exchange and then wire transfer it to their bank account or even directly send XXX.BTC to their BTC wallet with gateways wallet integration like in OpenLedger. Once people have converted their SBD for XXX.USD/BTC, SBD will most likely end up in the hands of market makers who are responsive to incentives and arbitrage opportunities, and will use any incentive mecanism thrown their way by the blockchain to convert SBD back to Steem.