I agree with you on a few points:

1. Stable power grids are much easier with a mix of generation sources that includes substantial rotating mass and baseload generators.

2. Nuclear is awesome from a climate change and energy security point of view, and it would be amazing if it were cheaper or more valued.

When power was primarily generated by thermal plants with big rotating masses, we got frequency control implicitly from the inertia of those generators. When there was a demand spike, the generators handled the millisecond to few seconds regime just by their inertia, while the seconds to minutes regime was handled by plant control systems increasing throttles or starting more peaker plants.

I disagree that renewables themselves are the problem. Cheap solar energy does not have to mean that we shut down all the uneconomical generation sources, nor does it mean that we cannot do FCAS with modern technology.

Battery electric storage systems have actually eaten much of the FCAS market in the USA, where they can respond way more effectively and efficiently than other systems in the 1 to 10 second regime. By and large, we don't store solar energy for use overnight - we store it (or really any energy) for use in smoothing short demand spikes.

I would love to see more nuclear, and more advanced nuclear. Modern designs are safe, effective, and amazingly capable. They just aren't as cheap as paving the world with solar cells or burning natural gas left over as a fracking byproduct.