It is important to consider that not all units (either delivered or proposed) have similar electrical efficiency. In a curve like indicated above one is in a better position if the efficiency (chip efficiency, PSU efficiency, and individual electrical rates) is superior to the "average Joe". As difficulty climbs the margins on the marginal miner (higher electrical cost in USD per hash) will be crushed first. Those miners will go negative and once they are sure that price/difficulty isn't going to improve eventually idle their rigs. That means it is possible for hashrate to decline independent of price, of course any decline will be short lived as miners calculate higher ROI% on new units and place orders. Unless shipping time is 1 day though there will be a lag (we have seen it in the GPU era even with supplies essentially unconstrained) and those higher efficiency units will be able to power through.
Still there are always risks involved. The odds are nobody is going to make a fortune mining anymore. I would be happy to just generate some coins at a small margin and reduce the needs to deal with awful exchanges.
Another important point: by the time an Avalon unit is no longer electrically efficient the difficulty will be around 2-5 billion, depending on power costs.
At that point this HashFast unit will be making only about $8-4 a day, even if it's just 100 Watts.
So, while that's going to be an issue in the long run, there's still a lot of profit to be had on the way up from 300 million to 2 billion difficulty.
That's why KnC is a better bet
right now even though it's more expensive. I think it's pretty likely that a KnC unit delivered in September or October will at least cover the
cost difference between this and their units.
That said, I do think HashFast will get a lot of sales anyway.
If HashFast wants sales, they should tie the hashrate of their miners to the difficulty rate on delivery. I.e. so you buy the equivalent of 500TH/s today, if the difficulty goes up more then expected, they'd send you more hash power.