Etherium is mined by GPUs at the moment. So its Etherium that needs to crash for GPUs to become available on Ebay. Etherium has been crashing too, but it seems to be a little bit more stable than BTC actually.
So keep your eye on Etherium prices instead. Maybe Monero (supposed to be a CPU-coin, but some GPUs mine Monero just fine). Those are better representative of the GPU-mining market.
For example sovereign issuers of fiat currency insist that taxes are paid in their fiat currency.
Because that's what really makes a currency once we get past the facile "a unit of exchange" chat.
You could find a shop that takes bitcoin. You buy all your food there. Life is good, you are sat with your girlfriend eating croissants from this store. There's someone at the door. It's the government. Taxes time.
Your point about the state having a monopoly on violence is critical. They want their taxes, they will send you a letter. Then another letter. Eventually they send a guy with a gun.
I don't necessarily have a problem with the state having that monopoly. I'm not some raving libertarian. But the failure of bitcoin advocates to understand this distinction is wearing.
About $20m yesterday.
You have to subtract the transaction fees:
https://charts.bitcoin.com/chart/transaction-fees-value
About $2m yesterday.
Until 2020 this is the formula:
12.5 * 24 * 6 * price