I think that Steem Dollar is only more expensive than Steem because there are less SBD out there than "regular" Steem. However, the number of SBD is (if I am right) infinite. Means => somewhen there will be more SBD than Steem out there and then - at least I think so - Steem will be worthier than SBD on long term
SBD can be destroyed by converting them, ie calling the debt and receiving the steem in exchange. Realistically this will only happen when the price of SBD is less than or equal to $1, so for the near term we will have an expanding supply of SBD.