Yes, coal is becoming less economical. But some caveats to the headline for the USA (based on reading the report and a few other sources):
The $78 billion is the cost savings is basically to the electrical industry, not the government. The savings would be passed on to consumers or shareholders, not end up in government coffers.
It appears that if you want to include subsidies, coal gets far less per MW than renewable. Switching from coal to renewable decreases the pool of money the government has.
The $78 billion is based on keeping the plants going for 17 more years and is a comparison of trends and spread out over time. Plants are already being closed, so this isn’t an additional $78 billion. This is a calculation of what the industry is predicted to save if they follow trends of closing unprofitable plants.
In short, don’t get too excited about some windfall if we act quickly.
Also, keep in mind that there are many reasons to keep an unprofitable plant running. Closing a plant is not a zero cost. Changing to another source requires immediate capital investment. Simple cash flow can limit how fast you change over. There’s also risk mitigation. Having some coal or nuclear generation capacity hedges against a spike in NG prices (if, say, every power plant were to suddenly switch).
Also, the success of renewable energy works a bit against it. Prices are dropping nicely. So should you convert over now, or take a loss for a couple years in expectation of more than saving that much from the drop in solar and wind capital costs?
Lastly, don’t confuse the electrical industry with the coal industry. Power companies don’t give a rats ass about coal. Or solar for that matter. They care about money. You can be pretty sure your power company is doing the same calculations as this report. The $78 billion isn’t even really a savings. It’s probably closer to a projection of what it would cost to subsidize coal to keep at the current level.