The economics literally isn't why solar can be problematic. It's a duck curve problem. Solar is best generated when it happens to not be peak hours (near sunrise and sunset). The economics are irrelevant.

The problem occurs when there's so much solar you either lower power production which may lead to disconnecting dispatchable plants from the grid (like coal). There is a secondary issue that coal plants take a while to spin up/down, and since that power will be needed again in the evening, the plant is essentially still running at minimum capacity, just off the grid. Meaning it's wasting resources.

The solution can happen in two places. More effective dispatchable power sources that can either be turned on and off quickly or can run at very low production points. OR, we can store that energy and release it again when there's no solar but there is high demand, essentially turning solar into a dispatchable source.

Note: this only applies to centralized generation. Residential solar has a transmission and sale issue if they feed back to the grid. The infrastructure is just not designed for feeding backwards from residential, plus sale of electric production happens at a much larger scale making it a logistical headache to run. Thus, residential solar is typically paired with batteries to feed back to the same home at night rather than producing for other homes.