Suppose you need 10GW of power for an absolute baseline. Enough to heat homes to a temperature that people don't freeze to death on a cold day, to keep power to hospitals and other critical services, etc. Then you need another 10GW on top of that to run aluminum smelters and heat homes to 80°F instead of 60°F and things like that.
If you have 20GW (average) of wind but you get an extended period of low generation and the batteries run down, people die. If you have 10GW (average) of wind and 10GW of nuclear and you get an extended period of low wind generation, the price of electricity goes up that week and people turn off their aluminum smelters and things but nobody dies. If you have 20GW of nuclear you can run the aluminum smelter 52 weeks a year instead of 51 but then people are paying more for electricity than they would with renewables in the mix, which isn't worth it.
Because it's not that simple. If you want 100% availability year-round then you need about 2X overproduction and quite a lot of storage, not just the four hours normally paired with solar today. That could end up being more expensive than nuclear.
But that doesn't change the fact that solar on the margins, without the availability requirement, is quite a bit cheaper than nuclear. So going 100% nuclear probably isn't the cheapest option either. The optimum is a mix in the middle somewhere.
If that’s too expensive why not just build enough nuclear to cover it all.