Have you thought about the amount of money needed for last year's network hashrate jump and the money needed for next year's jump? It's a big big difference. Jumping from 150TH/s to 120PH/s is not the same as a jump from 120PH to 3600PH/s. If we assume a 0.5$/GH * 3480PH= 1.74B$ needed for this diff jump. Don't really think that there are so many money that are just waiting to be spent on bitcoin hardware.
Difficulty only needs to increase fivefold before electricity costs eat up half your earnings. So you would only need to add ~500PH or $250M.
Sure. Been there done that already, don't worry. By the time the network will add another 500PH I will have better and more efficient equipment. Since I don't see AMs 60PH being sold so fast as you expected I'm sure that it will take some time before adding another 500PH.
Good point. At today's BTC price, there's only about half that $1.74B of value to be mined in the coming year.
You can't assume $0.5/GH will be the going hardware rate for a whole year though.
I would be interested in making a small wager about what the difficulty will be at the end of June 2015!
Sure. Let's have a 0.1 BTC bet. You bet on your desired difficulty at the end of June and I win if it's less and you win if it's more.