It doesn’t matter how much solar you build; without storage you’ve got zero power available at night.

The issue with overbuilding solar is that you drive daytime electricity prices to zero so that everyone is losing money on all these solar plants. Furthermore, base load plants such as nuclear plants also start losing money and they have no ability to shut down during peak hours. So you end up driving the base load plants out of business and they shut down permanently. Now you have even less capacity available at night! This causes nighttime power to become extremely unreliable, potentially leading to rolling blackouts and skyrocketing nighttime energy prices.

Another issue that people rarely discuss is the quality of power on the grid. All the grids in the world operate on 50/60 Hz AC which must be carefully maintained at an accurate frequency and synchronized with the grid. The main base load turbines are the source of this waveform which is carefully monitored and adjusted to remain stable.

Solar panels produce DC power which needs to be converted into AC with an inverter and synchronized with the grid. The problem is that if all the base load turbines are taken off the grid then there is nothing for the solar inverters to synchronize with! Turbines are nice and stable because they’re literally an enormous, massive spinning flywheel. Without them you’ll have an extremely unstable system where all of the solar plants are trying to adjust their frequencies and phases to match each other and the whole thing wanders all over the place.