But entrepreneurs take risks too I'm only highlighting the situation that a bad investment will still never ROI but continue to support Bitcoin by mining.
Also the network is already more secure then it realistically needs to be right now based on the transaction volume
lets say I spend 10BTC (cash - no interest due) to buy a few miners. That is a sunk cost.
If I never get a return on that 10BTC by mining (ie. A bad investment) it is still possible to carry on if my operating costs are less than my income.
At the moment many miners have an operation cost around $130CAD per BTC, some won't earn enough to recoup the investment in hardware, but given the hardware is fully paid, quitting now when the price is $380CAD is premature. They are in the red but plowing on is better than throwing $250 away by stopping.
In a way you are right, but you should also think of the resale value of the miners. If your net profit is really low then you should actually sell off the miners when the value is higher.
It also seems to me that bitcoin miner value drop about 1/3 every month. So if you think your profit in a month is less then 1/3 of your hardware price then you should consider selling it off...