If you're paying $80/MWh for captial and fixed O&M and $20/MWh for fuel and variable O&M, then you're still payiny 80% for the energy you don't produce.
And with wind and solar, you need storage, which costs a lot and similarly is is not used most of the time.
That said, in New Zealand the story is different, because we have some very large battery banks called hydropower lakes. However when our batteries run dry, the country has a bad time.
Currently storage is over 3TWh[1] and in 2020 NZ hydro generated 24TWh[2].
Their point is interesting. If you already have hydro and add solar or wind, can you pump back into the lakes during periods of excess, and use the hydro at night?
Yeah this is a thing. If you have enough hydro you can just turn it off entirely when the renewables are going and have a 50/50 mix.
If not, it's called pumped hydro storage (or most precisely it's blie field on river pumped hydro if it's already dammed and on a river). You need to trap the water somewhere so you don't have to pump it too far. This involves building a lower reservoir and adding pumps or modifying the turbines to be two way.
At that point it's effectively just curtailment.