We started off with 100% of power generation being either fossil fuels or nuclear (+hydro etc. but these are minor for most places). As part-time renewables enter the stage, they take priority over the fossil fuel plants when they do produce, and simply go quiet when the wind don't blow and the sun don't shine. We accept that, because we're fundamentally still in a fossil fuel energy mindset.
So what does the post-fossil fuel world look like, with a nuclear+renewables mix? Well, we have as much renewables as we'd like, but really (and here's the kicker) we also need enough nuclear for windless nights (or winter days which in e.g. Northern Europe barely count as daytime). So really, we need to be able to power nearly 100% of the economy through nuclear alone in those cases.
But then, that begs the question, do we still need the renewables at all? Right now it probably makes sense, because it lets you throttle back the fossil fuel burning, so is a net win. But if your baseload generation is carbon-free (at the margin anyway), it doesn't matter. You just don't need the renewables.
This is somewhat oversimplified, perhaps. E.g. the times when you are less likely to have wind and solar are at night, when energy use is lower (though will that remain the case when everyone's electric car is then charging?). Also, winters drive heating demand. Perhaps some small amount of storage makes a big difference; e.g. if we could store a week's worth of power, that takes us through all but the biggest troughs in power generation, and you need less nuclear (though again, not sure how that works with winters).
I'm not nuclear-crazy, if we could do solar+wind+batteries, why not. But I find the renewables lobby just says "yeah prices will fall, we will solve energy storage, just go all in on renewables, perish nuclear", which just seems like crazy wishful thinking. Whereas nuclear is always described based on 1970s technology and with the most exaggerated (un)safety numbers.