The big mining farms can still be very profitable. Tax write offs are not needed.
If you pay under $0.06 cents per KWh, and are an ASIC manufacturer, you are still making good money from your massive farms.
The current return on mining is 18 cents per hour Terrahash, or 18 cents per KWh, well above running cost of say 6 cents per KWh.
This assumes mining efficiency of 1J/Gigahash and $300 per bitcoin. In fact the recent miner hardware is significantly more efficent than 1J/gigahash and large farms in places like Iceland, Sweden, Washington State and parts of China are likely to be paying less than $0.06 per KWh.
While home mining in most places is unprofitable and miners have been turned off in droves don't assume everyone is making a loss.
Ultimately the difficulty will keep slowly rising till the most economic miners are marginally profitable but we are not there yet. And uneconomic large mining farms will not be dismantled immediately, instead they will be mothballed, waiting for more profitable times.
I posted an article about Chinese power prices a few days ago. Nobody in China is getting 0.06 power. Where there is cheap power, operating costs are higher with the possible extension of the Republic of Georgia which has large untapped hydroelectric potential. I think at least one farm is going in there despite the potential geopolitical issues. In the Pacific NW of the US, BPA delivers power at 0.034/KWH. Then there are transmission costs and losses, taxes and whatnot added onto that before it gets to the mining farm. Then you are paying US scale labor, property tax. IIRC, WA is a no income tax state but you pay property tax on your capital stock. All of this adds up. I suspect no mining operation has a marginal cost of operation of less than 0.10/KWH.
So, at $300, 10% bumps every 13 days, and 0.10/KWH cost of operations: the 1W/1GHS machines are netting $1.95/day today and go negative on power in December. That means a large part of the 2014 hardware investment is a negative. Only the newest most efficient machine cash flow and then for only a few more months.
BTC prices are going to stay low because it is pretty obvious some large miners have to mine and sell. The public should be buying BTC instead of hardware at this point.
The impact of this is not so much on the hashing investment in the pipeline but on any ASIC manufacturer contemplating new investment.
I do think difficulty increases are going to slow down - perhaps by a lot. Good for me because power is not metered at the apartment in my building so my 1.5KW mining operation goes unnoticed.