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Thinking more about the zero-sum mining thing... If Bitcoin's market cap is higher than the total sum spent mining (has anyone measured this?), it might be because the exchange rate has gone up since most of the coins have been mined. Coins which were sold and THEN gained value would be speculation profit, not mining profit. I'm assuming price drives difficulty but not vice-versa.
The main point I'm trying to make is if mining was positive-sum, more people would mine (reducing profits for other miners) until it became zero-sum.
By using your logic, *every business becomes a zero-sum game*. If making widgets is profitable, more and more people start making widgets until the profit margin becomes zero. Therefore, widget manufacture is a zero-sum game. (widget = anything you want, a placeholder, fill in the blank)
With expensive hardware it makes sense to mine at a slight loss sometimes, since hardware is a sunk cost.
Yes, sometimes. It's like putting up a buy wall in hopes of stemming the sell tide. Anyone not heavily invested in the currency they're mining at a loss is a fool or a good Bitcoin Samaritan. Call me jaded, but i don't like the odds with either one.
So if we're in an ASIC bubble, could the market be temporarily negative sum?
No.