My sense is that coal mines 1) take a lot of money to make in the first place and 2) have poor future prospects. So the thing that happens if you buy a 40-year old coal mine with 10 years of coal left, and shut it down instead of operate it, is not that someone else just opens up a new coal mine with 50 years of life on it. [But this is probably more of a local effect than a global one—people are actually opening new coal mines somewhere.]
My sense is that coal mines 1) take a lot of money to make in the first place and 2) have poor future prospects. So the thing that happens if you buy a 40-year old coal mine with 10 years of coal left, and shut it down instead of operate it, is not that someone else just opens up a new coal mine with 50 years of life on it. [But this is probably more of a local effect than a global one—people are actually opening new coal mines somewhere.]