What cannot be sustained indefinitely is a central bank raising the value of its currency against external market pressure, unless it has the full co-operation of the central bank of the other currency in the exchange.
Because the direct market action to raise the value of a currency is to buy the domestic currency with foreign exchange, and the central bank can only use whatever foreign exchange it has on hand.
However, a central bank operating under a reserve banking system can depress the value of its currency indefinitely, since that only requires buying foreign exchange with domestic currency, and the central bank has unlimited power to generate new domestic currency. I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.
