You need a mix. Solar is great during the day in nice weather. It's useless at night, it's significantly reduced in bad weather, and it's completely inelastic to demand. If the population of a large metropolitan area turns on their air conditioner for an unexpectedly hot (but cloudy) day, you can't just generate more solar power. Pumped storage reservoirs can alleviate that demand to some degree, but we would need to flood significant new chunks of land and invest in relocation of humans/wildlife and construction of new dams and hydro plants. Combining solar with wind power, geothermal, and biomass can help to mitigate the issues of each solution alone. It's also just a good idea to have multiple systems in place as it makes the entire thing less prone to catastrophic failure.