alt Hacker NewsNatural gas and hydro are firm power. You're correct that nuclear is not a good backup power source, but that's not the suggestion. The question is whether adding high-capacity-factor firm generation reduces total system cost. In regions where hydro and geothermal are unavailable, nuclear is the next best low-carbon option. I think you're underestimating the costs and difficulties associated with 100% renewables, especially transmission capacity between regions.
I think you're underestimating the costs and difficulties associated with 99.99% renewables and/or using it as a straw man against the highly inexpensive and achievable 95% renewables.
You're also underestimating the costs of using nuclear to achieve 99.99% reliability on top of a grid with 95% renewables: in that scenario you need nuclear capable of supplying ~100% of your power. In which case you might as well ditch the renewables. But the world does not have the quadrillions needed to go 100% nuclear. The world does have the 10s of trillions needed to go 95-99% renewable.