Thermal storage needs to be quite large though, at least with the stone/brick like mass they used back then. And you need to isolate it, otherwise you have no control over the release of that stored heat. I wonder if new materials, maybe something that undergoes phase change in that temperature range, could be a lot more space efficient.
And don’t forget that the plants are really expensive. Having them produce very little or even no power for half the time doesn’t help that at all.