What you're not taking into account is that hydroelectric power production facilities are currently sized only for the gravity-driven amount of water that reaches their reservoirs. Without making the reservoirs any larger, or adding any additional dams, you can simply add lots more turbines and pumps to the existing reservoirs and get a lot more power production out of them.

Consider that the vast majority of the pumping would be to even out the daily power cycle (the "duck curve"), whereas reservoirs are sized to hold years' worth of water. The amount of water pumped back uphill during the peak solar output of the day would be a negligible amount of water to the overall reservoir, and then you'd run it down through additional turbines in the evening to produce power.