I'm thinking collateral could be used to provide a floor in the price. There doesn't necessarily need to be a downward peg mechanism. What also hope is that witnesses could redirect Steem from the reward pool to back certain SMT's as a matter of policy. SMT's by their nature are inherently inflationary, locking up Steem as collateral would counter this, especially if it's directly from new issuance. Eventually it could completely encompass the Steem reward pool. If SMT's and Oracles succeed then the reward pool eventually becomes redundant, but it needs to be done in a way that doesn't create de facto inflation, but also ensures the tokens (both Steem and SMT's) have real value.