That could also be assuming people only mine direct to cash.. as long as its profitable (which it still is) all you have to do is cover the electric and wait for a sell opportunity later down the line.
I mean I could mine 0.1 ETH in a few weeks but why sell for yesterdays price of £1470, when I can set a sell limit on it for £1800? This way if it triggers (this is just an example btw) my order I have actually sold my ETH for £180 as opposed to selling it on the day for £147. When you observe it in this context, all a miner has to do is mine it till the sell time equates to the same daily profit as a few weeks ago!
If mining isn't profitable though, you're better off buying at today's price than mining.
i.e. you could mine that 0.1 ETH for £160, and like you say you have faith in the market, and sell at £180, for a £20 profit. Or you could just buy at £147 and using the same sell order you'll make £33.
It never makes sense to mine at a higher cost than the production cost.