Natural gas is cheaper at the moment. These prices fluctuate around based on matters of supply and demand. Recent years, yes, coal has been more expensive, but the industry has been faced with a large and growing web of red tape, essentially taxing and regulating up that cost for coal, much like what has been going on over the years with out soaring costs for health care, and education as well.

What Trump is saying, with the “war on coal” being over, is that the regulations that have been taxing up the cost of coal over the years, are largely in the process of being wiped away, those that havent been abolished already. This takes time to filter through the market, but what will we see from this? Eventually, MUCH lower cost for producing coal, and thus much lower market price for purchasing and using it.

So natural gas may be cheaper and more efficient now, but this is not guaranteed to always be the case. Plus look at the financial statements of the major nat gas producers, very heavily loaded down with debt, none of them are anywhere near profitable at current natural gas market prices. Throw in the federal reserve increasing interest rates, like they did to push the economy into the last 2008 financial crisis, and even a couple of these nat gas producers being pushed into bankruptcy, no way could or should we expect nat gas prices to stay as low as they are. Meanwhile, he cost of producing coal will be going down.

Tldr; nat gas is cheap at the moment but things change