Baseline supply doesn't need to exist. You just need supply to match demand. Which mix of plants produces that can change dynamically just fine. The 'baseline'

Baseline supply doesn't need to exist. You just need supply to match demand. Which mix of plants produces that can change dynamically just fine. The 'baseline' idea exists because of nuclear's high fixed costs and low marginal costs. This is also a form of generation which is complemented by dispatchable generation and storage, just the same as renewables, and does not complement renewables usefully at all. So if you wanted to build a grid based on nuclear you'd need to either overbuild it or build lots of storage, just the same as with renewables. And renewables absolutely destroy nuclear on cost.