Balance is what determines the supply mix else everyone would just run nukes. Previous commenter is right about why fossil fuels are still used, we don't have tech to replace their capabilities, which are necessary for reliability of the transmission grid. Energy storage is an area of huge investment right now because of this, with batteries and flywheel storage pilot projects to try and mature this technology. SMRs are another area of research. Programs like demand response to incentivize heavy consumers to change their usage patterns.

Without the ramp rate of fossils to respond quickly to grid conditions, there would be constant frequency drops and spikes across the transmission grid. Turbines would become out of sync from the frequency on the lines and things would start tripping and we would have a blackout. This is even more complex with unpredictable renewal integration where fossil becomes even more critical for its capabilities, while slightly less for its capacity.