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.
>in that scenario you need nuclear capable of supplying ~100% of your power.
Where are you getting that from? Again, nuclear doesn't need to be a full backup, and nobody is suggesting we ditch renewables for nuclear. A perfectly viable solution would be a grid with a majority of power coming from renewables and storage, with enough nuclear generation to reduce the amount of overbuild necessary and to stop burning natural gas.
>Individually, each firm technology delivers substantial cost reductions relative to portfolios restricted to wind, solar, and energy storage alone. Additionally, because each technology occupies a distinctive functional niche in the electricity system, having all of these technologies available optimizes the utilization rate of each resource and reduces system costs by up to 10% relative to cases with just one class of firm resource.
If you're using nuclear as a backstop in that way, you're using the same statistical techniques that let you use cheaper, less reliable sources of power for backstop.
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.