NBT is only as stable as USD, you are right. However, USD is vastly more stable than BTC and is still being used as the world standard.
Think of it in market caps. When the dollar falls, the market cap for NBT stays the same because it is in units of $1. NSR, however, is mostly recorded in terms of NSR/BTC, so it will most likely go up if the dollar falls. This will increase the NSR marketcap (also in units of $1) and allow us to print more NBT, thereby increasing the NBT market cap to match USD inflation. This works as long as USD doesn’t move too quickly.
We can always switch our peg to a different fiat if USD is no longer the world standard. We could also switch to basket of goods. However, for the moment, NBT pegged to USD should work fine.