No government guarantees traditional currencies either. This is why the purchasing power of traditional currencies has been allowed to erode over the long term. Governments coined (excuse the pun) the term inflation to fool the masses and explain away this erosion of value.
Inflation is just a euphemism for the reduction in value of currencies. The price of goods and services appears to rise because your dollar is worth less each year so you need more of them to purchase any given item.
For example, in my first year at school in 1971 I could buy a meat pie at the canteen for 12 cents. That would now not even buy the sauce to go with it. Production of meat pies has not become less efficient over the past 47 years. You should not now need a couple of dollars to buy a pie.
The Australian government has not guaranteed our currency (and nor should it). The fallacy that it does is not a valid argument against the use or value cryptocurrencies.
