It doesn't matter that not everyone is a "economically rational agent". What matters is that there are some of them. People who will reinvest their profit into more mining, to profit more and reinvest it further. The people who are currently fabbing their own ASICs and setting up mining farms in places where electricity is cheap. These people left unchecked will, over time, suck up all spare electricity generation capacity and eventually outbid quite a lot of normal electricity use.