There's nothing particularly special about USD. It just happens to be a convenient reference that shows pretty reasonable short term purchasing power stability (so if you have to pay your rent in a week, you don't risk not being able to because STEEM or BTC dumped off by 30% or more). It is also the most widely used and widely recognized currency internationally, so it makes sense on a global platform to use that rather than some other existing currency.

If the USD were to ever really go off the rails, or even if we just wanted to make a change, it is perfectly possible to peg SBD to something other than USD, such as gold, silver, or a basket of commodities. (The Steemit clone/offshoot site Golos uses gold and has Golos Backed Gold, aka GBG, instead of SBD.) All it takes (aside from consensus to make such a change) is for witnesses to update their price feed scripts to pull from different data sources. Very plausible.