It depends if anything distorts the market. If not, reduced demand should lead to temporarily reduced prices, then reduced supply and stabilisation of prices at a similar level. If demand keeps reducing, the same should happen, but the economies of scale are slowly lost, so the price goes up from its historical low.

Realistically, there will be some distortion, so it may be that the price paid for coal varies a lot in different parts of the world based on policy, lobbying and existing infrastructure. I think coal is clearly on its way out in Europe though. It’s already becoming an issue for things like preservation steam railways here, with alternatives being explored to deal with the effective death of coal.