If coal supply was really becoming limited you would see most extraction locations receiving huge profit as they reduce supply just like oil due to lack of completion. (It's a simple question of discounted future cash flows and competition.) However, the price of coal is still dominated by the cost of extraction and shipment across the board.

Fossil fuels are really mindbogglingly cheap, but you still see completion such as: Ethanol fuel in Brazil, nuclear power France, and by looking at the subsidies it takes to adopt wind power you can see how close it is to coal.