Building larger heat storage capacity, for example, would be useless if it cools down before being used. Long-term storage is an area of active research. There is reason to believe[1] that the technology could improve in the future, but until that is proven to be viable the state of the art for duration is "a few hours".
Speculation doesn't keep the lights on. The grid can only be powered by things that actually work in the present.
[1] https://spectrum.ieee.org/long-term-energy-storage-molten-sa...
> [15] Ehrlich, Robert (2013). "Thermal storage". Renewable Energy: A First Course. CRC Press. p. 375. ISBN 978-1-4398-6115-8.
https://en.wikipedia.org/wiki/Thermal_energy_storage#Molten_...
> The well-insulated tanks where molten salt is stored contain enough thermal energy for up to a week
A false claim. It has enough energy for 7.5 hours. (149.7MW/1123MWh)
In any case, is says nothing about duration. The wiki writer may have just mistaken capacity for duration. I couldn't find any evidence of this power station storing energy for any amount of time. It's used on the night immediately after the day that charged it.