The same people that build nuclear don't build solar or wind. And yes, there is a huge shortage for these people with renewables, which is where the black and white flat cost of energy sources breaks down. Renewables are slightly less expensive than nuclear, but infinitely more expensive when their natural source is unavailable and battery / hydro storage is depleted. "far too expensive" is highly over exaggerated when nuclear costs about 1-3x as much as renewables, with newer reactors being on the low end of that scale.

On top of that there are very few moments where there is almost no wind or sunshine over a very large area all at once, making it economically unviable to an enormous degree.

To make this happen you need a massive amount of overcompensation for the times that the sun does shine and the wind does blow. The kind that isn't economically viable. You're building decentralized infrastructure that needs to be maintained while being essentially useless a lot of the time. You also can't exactly build a new wind/solar park in response to short term fluctuating demand, while you can scale up reactor utility.

Obviously the best backup is battery storage, ... And it allows far more decentralisation: small battery pack, which combined with some solar allows them to be basically off the grid.

The biggest users of electricity are not homes. You're right, this is a fine setup for houses. But you're not going to solve the biggest energy users this way. Not to mention, even for people at home, the amount of rare earth metals required with current technology is an ecological disaster in it's own right. We need batteries, but lets not pretend they are currently a final solution. Decentralization is not a magic cure all, decentralization also causes places with outdated energy infrastructure requiring new investments to completely revamp the system. This is not economically viable.

But even something like gas is a more preferable alternative to nuclear. It’s very cheap and still viable when needing to spin up or down quickly.

This is not carbon neutral, which is what our goal is. So you're essentially conceding the point here. You also highly overestimate how many days a year you would need them, considering the sun doesn't shine for at least HALF the day on average.

Unfortunately it is, because money is finite. And investers choose whatever is most viable, which increasingly is not nuclear.

Which is why nuclear is necessary. As the engineers that can build and maintain renewables are busy, and the grid is oversaturated with renewables when the sun is shining and the wind blowing (causing their efficiency / utilization to fall if you build any more) they will eventually break the equation in favor of nuclear. And there are still nuclear reactors being built in places where public opinion isn't irrationally afraid of nuclear.

The sad thing is we could have been building them 20 years ago, and have had massive steps ahead in being green now. Instead we are here hoping for some kind of miracle technology like cheap batteries, nuclear fusion, carbon capture, all which isn't a certainty to become fruitful, yet nuclear is here right now.