POsts are not rewarded with steem.... They are rewarded with SBD and vests.
In response to those rewards, steem are created and placed in the vesting fund. Those created steem are what backs SBD.. they are created by mining, so i suppose you could say theyre created by nothing.
Any profits you get from an increase in STEEM market cap are socialized by destroying some of the backing STEEM, essentially all other STEEM holders split 100% of "your" gains.
You got this opposite... a decrease in the steem marketcap actually gives you profit with SBD, but yeah youre right. The loss is socialized as a loss to the vesting fund.. Because when SBD are redeemed, they are taken out of the vesting fund. thats why if enough ppl convert SBD< the steem represented by your SP balance would become lower.
All an SBD is is a debt instrument drawn on the vesting fund.
Thats why (and this was the initial bone of contention) the real measure of SBD debt exposure to the system is a comparison between SBD supply and vesting shares. Becuase SBD redemption comes out of vesting shares.