Steemit does not back Steem Dollars and never promised to back them. Steem Dollars are backed by the blockchains ability to create new Steem. Those who hold Steem Dollars are like investors lending money to a startup using a convertible note.

In effect, those who hold Steem Dollars are lending the value of a dollar to the blockchain secured by the ability to print Steem. In exchange for lending the blockchain dollars, they are paid interest in the form of more dollars.

If the value of the blockchain were to implode to near 0, then all Steem Dollars would get converted to Steem at the same ratio and existing Steem holders would lose almost everything. Steem would be under new ownership (Steem Dollar creditors would take over the Steem).