Its not that you did the math wrong, its that your valuation of coins has remained constant.
And we are straight into another problem with your calculations. You keep mixing USD and BTC at rates picked out of thin air. Everything kan be made looking profitable with enough invented numbers.
To compare buying 1 share at 1 BTC to just keeping the BTC, you need to do all those calculations in BTC. The price development of BTC will decide if both are profitable. If you can deliver more BTC back than I pay for the share, your model will be more profitable than just keeping the money.
Doing the calculations in BTC is impossible because BTC prices change too quickly. Yes, I'm well aware that collecting 1m BTC may end up with a $3 or $7 million dollar IPO instead because of this, the plan is flexible enough to deal with that. However, until I can buy property and FPGAs in Bitcoins and pay taxes in Bitcoins and go to the corner store and pay with Bitcoins, what you're saying is wishful thinking... I look forward to the day that I can, but Bitcoin isn't there yet.
USD is a factor and we can't ignore that; otherwise we end up with another 10k BTC delivery pizza.
Continuing investment in new mining technology is a valid point, but it makes the constant difficulty assumption even less valid. Even the valuation of BTC if you grow to much, because you could pull off a 51% attack. Even if growth beyond your initial 1/3 of the hashrate would only be stupid unless difficulty increases, because at that point you mainly compete with yourself. Your share of the blocks will increase slower and slower the higher percentage of the total hashrate you get, and you will push difficulty higher. Many GPU miners, like me, don't need to mine for profit. It is enough to get about 2/3 of my power costs back, and it is still the cheapest heating system around. If price per block and difficulty remains constant, as you assume, GPU mining will never become unprofitable.
If the global hash rate is 10 thash, that doesn't mean that a 51% attack is 5... it means its ANOTHER 10. And even then, its not guaranteed (ask gmaxwell sometime on the exact numbers, its pretty difficult to reliably perform the attack even with that 10 thash).
That said, yes, I will purposely back off if we get close to 50% because, frankly, its insane and its bad for investors. However, increasing diff to price out most GPU users IS good for investors. Nvidia and Radeon 4xxx have already crossed the points of no return, it is only time for even the most efficient GPU setups to fall behind, even without DMC. Being aggressive for a large scale mining operation isn't a bad thing.
As for GPU mining becoming unprofitable, all you're doing is calculating cost of heating into your total operating cost math. Which, don't get me wrong, thats pretty smart... for the 2-5 months your part of the world calls Winter. The other months you're turning the A/C and/or shutting mining off. That is not something a large scale mining operation should do.
Diff and BTC price are not constant. Given the entire history of Bitcoin, on average, diff is going up quickly and BTC price is not significantly increasing. For DMC, or any mining operation, worst case scenarios must be planned for. Even paying for electricity at industrial rates (2 cents kwh) is a liability.
In addition, the DMC shares themselves might go up greatly in value if I can swing power generation profits far enough in our favor to make it an important part of the company and not just a way to stave off ever rising power costs (but thats a very long term outlook). Dividends don't solely have to be generated by mining alone if we're already investing heavily in green power.
Green power isn't always profitable. Especially in the US where coal is heavily subsidized.
Its not unprofitable either. For us, we would have two income streams. Green power would be reducing our total operating costs, whatever is left is then sold to the grid: we can't keep it, the overhead of operating a battery array would just be too expensive. The other income stream would be the mining operation itself.
Coal is heavily subsidized because it is expensive to operate. We're in orbit around a gravity fed highly efficient fusion reactor that outputs more power in a second than Humanity has used in its time on Earth. Solar and Wind are the only power generation techniques that are cheap to deploy, cheap to run, and will never run dry.
It would be insane not to take advantage of this any way we can.