There's no need for a "backing", what matters is that it's impossible to "print money". Physical metal coins are only one way to do so, and not even the best (as they can be mined, and take up metals that could be put into useful work). Of course, current cryptocurriences are not that good, either -- waste of electricity, waste of disk space (Bitcoin currently needs 700GB to meaningfully participate), transaction costs, etc. Metal coins are merely the most traditional one.
Too bad, those in power (and money = power) have grown too attached to the ability to pull money out of thin air. Debasing of coins was popular for thousands of years, most countries defaulted in early 20th century (Germany 1914, UK 1925, US 1933) then successively removed remaining safety barriers (US went into freefall in 1971). And most of money from nothing is "produced" by banks rather than governments.
We are told that "inflation is good" by economists -- for me, it's nothing but a tax. Asking an economist whose very paycheck comes from inflationary measures is same as asking a christian Bible scholar whether the Bible is trustworthy.