No currency ever has been 100% stable. The exchange rate is how we perceive it in the modern economy, but fluctuation in commodity prices was the indicator of an stable, or not, currency when the gold standard was in place. That will be the indicator if bitcoin, or any other currency, achieve 100% use, which in itself is impossible. But there is no guarantee to stability, bitcoin doesn’t provide it any more than the usd backed by millions of soldiers does. Stability begins on the production side, not the monetary one; after a huge drought you’ll see prices skyrocket irrespective of the currency.