Even if not a day, maybe still not responsive enough.
Then again, maybe it gets you close enough that now you only need a smaller more doable version of some otherwise unrealistic gap filler like batteries or thermal mass or gravity mass etc.
https://www.babcockpower.com/wp-content/uploads/2018/02/exte...
I imagine all the new ones use it, and thus all the new ones can change their outputs in a matter of minutes, instead of hours. But that still makes coal one of the slowest sources to react to grid volatility, and sill basically useless to compensate wind oscillations, only useful for the solar cycle.
Coal's market share in the US has been eroding for 15 years and will likely fall below nuclear soon. It has been replaced by natural gas and, more recently, renewables.
The reason that we "worldwide" have been building lots of coal plants is that China has been building lots of coal plants. This is despite China also building world-dominating quantities of nuclear, wind, and solar.
The reason China is adding so much coal appears to be that provincial officials get to book coal power production as economic growth internal to their province. By contrast, the electricity from renewables in Xinjiang shipped over HVDC transmission lines does not boost their province's economic growth numbers so they do not want to use it even if it would be cheaper.
With few exceptions in the world, in a electricity grid the supply and demand of electricity have to be 1:1, and if wind and solar is adding volatility to that supply, another flexible power source suddenly becomes very profitable and useful.
Nuclear is costly to turn on/off, but coal, biofuel and natural gas isn't. So turning it on/off based on the shortage caused by sustainable sources is why they're coming back, have been my understanding