A couple hundred million people are required to pay taxes using it. That implies that there is some inelastic demand for dollars.

That factor alone obviously doesn't prevent every currency collapse, but it is a key difference between "government fiat" and "geek fiat" currencies.

The monetary supply is tailored to the needs of the economy to prevent volatility and create value. By being able to buy or sell anything and have the currency worth a predictable amount there are many transactions that can happen that would not be possible with a more volatile currency.