The whole price advantage of thermal storage is in the amount of stored power. It's very cheap to build insulated tanks. It's expensive to build large turbines, pumps and heaters, the things you need for fast response. Thermal batteries are already kind of iffy compared to batteries (or rather, where batteries will be in 5+ years), so if you skew that price by several factors by going for high power, chemical batteries are definitely cheaper.
In a fully renewable grid, you'd need longer term storage like thermal batteries. 3 days is the most common figure. The electricity price would never go negative, even without batteries- renewables just turn off too quickly. You can just unplug solar panels if they're generating too much power.
It also isn't true that turning off any power station can't be done instantly. The station has to be able to cope with all its transmission lines suddenly being cut, which is a forced instant turnoff. It's more that the startup time is large, and they think that by losing money now, they'll still be running later when the power price is positive again.
Now as storage prices drop the equation changes, but not very quickly as their ineffecency means you still need extra capacity and that capacity reduces the need for storage.