Not sure if that's fair. Sometimes the sun actually does shine, and while you won't get rich selling energy during that time, having electricity available essentially for free during some of the peak hours is worth something. Someone will eventually find a use for free energy. If nothing else, at least we can turn off all coal, gas and oil plants while the sun shines.
When the sun shines sure you can power down other power stations, but you still need to maintain them and have them around for when the sun doesn't shine. So when you have lots of intermittent energy it means that you actually cannot shelve your old technology - you still have to maintain it, but now you're paying for the maintenance over less capacity used. This is only one of the reasons why prices are going up in heavy renewable grids.
The UK is a grid that is adding lots of renewables and building some nuclear plants, so the site provides good data around the costs for the added transmission, storage and subsidies (for both renewables and nuclear) needed for a renewables heavy grid supplemented with nuclear.
Gas peaker plants are designed to be turned on and off regularly. They can go from cold shutdown to full load in less than an hour, with some able to do it in less than 10 minutes[0].
A decent bunch of that is going to be sheer inertia as you have to spin up a heavy chunk of metal. Keep that rotating (which is actually a good thing in renewable-heavy grids!) and it can be done even faster. Similarly, keeping some parts warm costs very little energy and allows even faster ramp=up.
But that's actually not relevant because battery storage is far more attractive for that kind of short-term demand. The renewable grid will need gas plants which can fill in the gap left when it is both dark and wind-less - which due to meteorology is trivially known 72 hours in advance, leaving plenty of time to prepare the plant for use.
