The SBD peg is not a "hard" one, meaning is not pegged with hard cash, with USD kept in a safe deposit. It's more like a "soft" peg, which was designed as "$1 worth of STEEM". So basically it's a debt instrument. The problem with debt instruments is that if you abuse them, the "collateral" is triggered. An unsustainably high SBD could trigger a downward spiral for STEEM.

On the other side, it's true that SBD may be "pegged" to whatever amount of STEEM we want to, that's why I think there is still upwards potential. So in theory (as well as in practice), SBD could be safely "pegged" to "$10 worth of STEEM", or "$50 worth of STEEM", as long as the debt-to-ratio numbers are not too big.