Yup, storage just like that has been built in Switzerland: https://www.alpiq.com/power-generation/hydropower-plants/pro...
California is mountainous enough to follow suit.
It's already cheaper to use batteries than natural gas peaker plants (the most expensive form of energy). The next phase of storage is to defer or replace transmission like upgrades (which are also super expensive). After that we get into daily cycle operations like giving small amounts of dispatchability to solar or wind projects.
And just like solar, batteries are getting cheaper far far faster than anybody ever predicted. What people used to consider the absolute floor in terms of raw material cost For lithium ion batteries is dropping all the time too.
However, the challenge for scaling up renewables and storage is not technical, it's going to be political. Utilities are not typical businesses that will just switch to the cheapest way of doing things. They are excessively political, and lobby a ton in order to influence how they are regulated (and thus how they profit off a captive audience), and they have been close bedfellows of fossil fuel interests for a loooong time because their combined lobbying power is so much stronger.
Oddly enough, just as hydro won't be able to scale even where it makes sense because of popular political opposition, we won't be able to scale renewables and storage because of entrenched interests lobbying Against popular opinion (renewables are popular across all of the political spectrum).