Rockstable's promise is not to peg it to USD forever; rather, the promise is to keep its purchasing power fairly constant across a basket of commodities. The USD is the most widely-accepted and most stable of all fiat, so it makes sense to just peg against it while it is stable. But once USD loses its stability, it won't make sense to keep the peg. Instead, independent groups will measure ROKS purchasing power across a standard basket of commodities, across different locations around the world. These independent groups will be there not just to measure ROKS purchasing power, but also purchasing power of the rest of stablecoins existing by then.