I think a better proposal would mirror @burnpost in trading the SBD for STEEM if the price of SBD is $1+, and burning the resulting STEEM, and either burning or converting (via blockchain convert function) the SBD if the SBD price is <$1 (some additional nuance is needed for dealing with the haircut rule, which I haven't gotten into here).

These could be two different proposals (one to trade-and-burn and one to burn or convert-and-burn) but that requires stakeholders to be constantly adjusting proposal votes in response to market prices to accomplish the same maximally-beneficial effect, which is probably worse.

Burning SBD when it is worth more than $1 is actually harmful to Steem stakeholders so I wouldn't support this proposal as-is.