Sorry I misremembered the statistic. They've been reducing coal usage slightly over the last 20 years, but burn 50% more natural gas. overall energy generation from fossil fuels is up over the last 20 years (since their first nuclear plant was shut down)
So capacity of fossil fuels went up, but I suppose natural gas shuts down while renewable generation is high so it doesn't use its capacity all the time - my bad
Yes, the easiest way to get a stable grid with wind and solar is to have natural gas capacity equal to what you need from wind and solar. Gas is not used when there is wind or solar, and is very easy and quick to turn on when needed.
In the future when more green gasses become available, bio methane, hydrogen or syn methane, those gas plants can be filled with those as well.
Renewables have picked up both the reduction in coal and the reduction in nuclear power.