Plus, for the value to completely zero out you'd need lots of large organizations who have lots of money (be it ones who've borrowed it or ones who are 'holding' it for others) to not only run out of said money but also be unable to pull in new people with get-rich-quick schemes. Miners and stakers would also need to completely lose trust in cryptocurrency.
So, to zero it out, you would need a global economic crisis so large that it educates pretty much every 'sucker' out there, which is pretty unlikely to happen right now.
See the bit about the "floor" in your post.
Ordinary crypto currencies like BTC & ETH can't go bankrupt. They never had any liabilities to begin with. The majors such as BTC & ETH have deep, global, 24/7 liquidity. If you can find a way to make all the liquidity pools dry up for crypto around the world, I suppose it will go to zero. Good luck with that.
There is no floor for stocks.
How much are Pan Am, Enron, Lehman Brothers worth today?
The utility of Bitcoin is its global payment network.
Bitcoin, on the other hand, is some electricity, and arguably a waste of it.
/happy gilmore
So zero then?