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There is no reason to “close” these plants. I can see an argument where a place like Arizona could use more solar investment and maybe it will pay off - maybe. Of course solar only works for about 1/3 of the day, on sunny days. You also need to invest in massive power storage (batteries) basically meaning you need 3x to generate roughly 2x-3x more power from solar than you need each day in order to store the excess for nighttime use.

But assuming best case scenario for solar, other sources of energy should be kept at least minimally operational and ready to ramp up if and when Solar goes sideways, nighttime, rainy days, winters, if demand issues cause spikes in energy use, battery failures etc.

Solar wouldn't work for the grid even in Arizona. For one, there's currently no way to feasibly scale up fossil fueled plants when needed.

For another, there's the issue of demand. Demand is different from total power needed in a day. Demand is how much power is needed at any given moment. As an analogy, think of a water faucet letting water flow out of a large basin. How open the water faucet can go (how fast the water can flow) is like demand in power. How much water is in the basin is like how much total power comes from solar panels and/or batteries in a day.

There's a yuge demand in the afternoons when everyone gets home from work and school at the same time. In the south (including southwest like Arizona) that means that offices are still running their A/C for the people who are still there, while many homes are also running their A/C to cool down as people come home. While the parents run appliances to do household chores they couldn't do while at work, etc. My solar inverters can provide 18kW of continuous AC power from my solar panels and/or batteries, which is almost always all I need for what my wife and I are running -- but she's retired and I work from home a lot. In other words, we don't usually run all of our appliances at the same time to do all of our chores in a brief period. And the AC (variable speed heat pump) rarely runs full blast because it's never really off (letting the house get warm in the summer) and having to rush to cool the house down by the time we get home (because we're always home).

Most people's homes don't operate like that, so my inverters wouldn't be able to all of a sudden provide all of the power they need every afternoon. And that's just the homes. With businesses and hospitals and such the demand is often too out of the park for solar inverters to keep up with.

Nope. The only way solar is practical is with decentralized solar like I have --- and even then it's practical only if all the variables are right for your situation and also if there's a dependable grid available for the times I don't have enough solar and battery storage.

You need 3X more generation AND you have to build a storage plant to store that for use at night and dark days.

The capital cost estimates for solar and wind almost always conveniently ignore that simple fact.

It's not well known that coal fired power plants must have energy storage, too. But they store the raw fuel (coal) on the ground BEFORE it is converted to electricity. This is so the plant can get through freezes in winter, winter transportation disruptions, and labor disputes. Coal piles can typically feed a power plant for 90 days. The biggest battery storage systems today (all demonstration projects) can store the output of a large power plant for a few minutes at most.