>But the decision "with my current fixed plant, do I mine this hour, or not?" is unaffected by anticipation. Either the expected-reward is enough to pay incremental costs, or not.
Some people use mining as a way to privately purchase Bitcoins, and/or are speculating that the value will rise long term. Some would rather pay $105 of power and not have a banking paper trail connecting them to their coin purchases (usually because they are kind of paranoid or doing something legally questionable) than pay $100 to purchase the coins on an exchange.
Also, if you've never mined, there is something kind of geek-magical about creating money with your computer.
Yes, sure. I've mined, and even mined for a bit at an occasional loss after hitting punitive over-baseline electric rates).
But it's farfetched to predict that everyone who was willing to do something when the bitcoin reward was X will still be willing to do it, in the exact same amount, when the reward is reduced to X/2. (Even if the goal is 'anonymous bitcoins' at above-market rates, they've still become twice as costly.) Demand curves slope down, supply curves slope up, exceptions are rare and not in evidence here.
Some people use mining as a way to privately purchase Bitcoins, and/or are speculating that the value will rise long term. Some would rather pay $105 of power and not have a banking paper trail connecting them to their coin purchases (usually because they are kind of paranoid or doing something legally questionable) than pay $100 to purchase the coins on an exchange.
Also, if you've never mined, there is something kind of geek-magical about creating money with your computer.