Wind and solar aren't that much cheaper (or actually usually much more expensive), when you include the batteries that are required for them to operate as the entire grid. Right now they can only displace SOME not ALL of the energy generation of a country (unless you have huge natural resources of hydro or geothermal energy that can act as that base load). Nuclear is needed for that baseload, or tons and tons of batteries. Taken in that respect Nuclear is cheaper than solar and wind, for that purpose. Grids will eventually need to go somewhere around 80% solar/wind and 20% nuclear/geothermal/hydro (+/- 15%).