You asked, "How are two generating systems cheaper than one?" When fuel costs are $0. It's cheaper to have natural gas on standby and use solar power when it's available. Over a 30-year operating life, using as much solar as possible is cheaper than burning natural gas to meet the entire demand. Our energy system is moving to battery-buffered solar first, with pumped hydro and natural gas as backup. Unless fusion happens in a way that reactors can make their own fuel, nuclear power is done.