I used to work for a drywall manufacturer who still owned their own mines despite efforts to divest from them by some. They always viewed it as a structural advantage to still own them and not be wholly dependent on the coal plants (which effectively have conveyor belts going from the coal plants to the wallboard plants). I imagine as time goes on it'll become even more of an advantage for them to still own those mines as their competitors are forced to buy at highly inflated prices (or even from them) as coal shuts down.