The value of cryptocurrencies, in spite of a collapse in value, is stopped from falling to their natural value, i.e. zero, by a combination of market manipulation, investor ignorance and a tsunami of lies on social media from those paid to promote these worthless frauds.
Which isn't how the US government backing the USD works at all.
Also note that I intend these as descriptive and not proscriptive statements. I'm not trying to say the government isn't wasteful (though 90-98% waste is an overstatement) or that it's the best way to do things, I'm just trying to illustrate why seemingly ponzi-like systems like national currencies can be economically productive and stable over long periods of time while truly ponzi-like systems rarely have longevity (Herbalife and Amway being exceptions to this rule, in my opinion) because they don't actually produce anything.