Wind and solar aren't free, no but coal for example is expensive to mine and process and transport and then you burn it and its gone. You have to mine and process and transport more and more and keep doing it forever.
Storage is still developing but the technology is advancing exponentially. Remember the first cell phone you saw? You needed a holster to carry the battery. Not too long ago a battery the size of a dime was science fiction. Lots of really smart people are working on the battery/storage problem and I'm convinced we're living in the last days of burning stuff to make energy.
It comes down to how much does each kWh of electricity cost to produce.
Coal does have a bad logistic tail, but it is cheap, and very reliable.
Wind and Solar have several costs, the up front cost, the land leasing cost, and the maintenance costs,
these have to be prorated into every kWh generated, just like the cost of every ton of coal burned.
The problem is that with wind and solar, the electricity may or may not be available when demanded.
With the coal plant, as long as you have a supply of coal, electricity can be generated to match the demand,
up to the capacity of the powerplant.
Technological advances are a double edge sword, they may improve the viability of wind, but the same improvement
may enable more efficient traditional power plants.
A battery broadly defined as an energy storage device, may not take the form of a closed chemical cycle battery.
I think storing energy as hydrocarbon fuel, may be a better sustainable appraoch.