I think you are confusing 70 PH/s "dropping off" with variance. No one can accurately tell just how many PH/s are currently hashing, and instead, those graphs use their best guess based on the number of blocks mined in a given period of time and the difficulty. So what happened was, for a short period of time, there were more blocks mined than on average, which made it seem like there was more hashpower on the network, now we are seeing less blocks mined in the same amount of time, because it all averages out in the end, and it is looking like a bunch of workers dropped off.
Also, the difficulty dropped by less than a percent, and the next difficulty so far doesn't look like it will be more than a couple percent in either direction. If 70 PH/s all of a sudden dropped off of the network, you would see a difficulty decline of over 25%.
Also, BFL had a rather large farm (a few PH/s) but no where near 70, they were not competent enough to maintain something that large.
I doubt a corporate farm would shut off tens of millions of dollars worth of equipment when they are still making money, and if your large enough to have 70 PH/s you will have backup power for this kind of situation.
There is no new coins, most newly launched coins are crap, everyone knows it, and they die rather quickly. If something was profitable, it would be very quickly mined into non profitability.