I read the article. And it's 13% per month difficulty is way off. The article stated difficulty doubling every 5-6 months. But in actuality, it more than doubled in under 3 months! I used the mining calculators in the last week of Nov. 2017. I took a snapshot of how much bitcoin an S9 makes and it was only 0.92 bitcoin back then. Today an S9 only makes 0.43 bitcoins. Regardless of the price of bitcoin, that's more than a 50% drop in under 3 months. If this continues an S9 won't make much bitcoins, but it can still be profitable if the price of bitcoin moves up exponentially. If the price stagnates, you can forget about making a profit even with so called "free electricity". If difficulty continues to shoot up exponentially, bitcoin mining will go the way of dash mining! So all cryptos designed this way will become unprofitable to mine eventually, once difficulty gets high enough.