POS=no production cost=no value
If I can produce a coin without any cost, why should I pay any thing valuable to exchange it??? I will just go ahead to mine it
Fiat money can have value without production cost, because there is a law to force its circulation, and the cost to make that law can be a war, which costs millions of times more energy than POW mining
Pos concept still needs electricity to keep the computer on for a certain time period before the reward comes. There are certain systems that dictates at least minimum 2 hours so there is still some cost of production attached to here