You are not taking into account difficulty as people bring more miners online. Even if the price stayed flat this whole time you would be making less.
Exactly this, in one word difficulty. Even at today's reduced profitability, as fanatic26 pointed out, it is still a lot better profit than many traditional investments. This means people are still going to be adding rigs daily.
You can expect some people to continue to add rigs even after the profitability is gone for you paying 15 cents, as there are a lot of people paying as low as 2-3 cents a kwh. Then there are also the 1000's of the smaller miners who pay no electricity from either still living at home with their parents, living in a utilities included arrangement (college dorms or some types of apartment rentals), people on some type of government assistance, etc. It is essentially pure profit for these types of miners as someone else is basically subsiding their income by making their electric payments for them once they ROI on hardware.
Many long term miners have been trying to warn people to not get in right now. With hardware prices still at well above MSRP and difficulty continuing to skyrocket, I am afraid a lot of recent newcomers will eventually be burnt. So while at this point in time the calculators are still showing decent profit, the next couple of months this will change for the worse. The only way out will be for coins such as Ethereum to not only return to $400, but keep on climbing so as to outrace the climb in difficulty. Plus sooner or later Ethereum is going POS, so at that point any miners left will be looking to jump on the few remaining mine-able coins thus repeating the whole process until we are back to making sub 25 cents a day per rig.
