Nuclear vs fossil gets in to why you don't/can't run all nuclear, else things would be very easy. Nuclear's capabilities are best suited to supplying the base load/minimum demand but they can't be ramped or dispatched, reactors basically run most efficiently at their designed output levels, so you can't use them to balance supply and demand. The use of fossils for base load is more a thing in countries with lower regulations, usually because of things like a growing manufacturing economy (ie "global south"), but also in some extraordinary regulatory circumstances (Germany) or just because of when fossil was brought online/refurbished. Fossil's capabilities are like the opposite and they are most efficient and economical used for load-following, which is even more important with renewables you can't dispatch.

So fossil is still the main control lever for reliability, and that's the crux of why a suitable replacement technology isn't available yet. If it was simply a matter of output level then we'd have no problem. Mitigations to reduce use of fossils when demand is high can even be things like a demand response/dr program for transmission-connected facilities, where they are incentivized to reduce their use during times of high demand. Basically instead of having a higher energy price and all this generation online, you take a bit of what that price would be and use it to incentivize consumers to reduce their demand. Smart stuff but fossils are still a thing with that and if storage could replace them we could easily just have nuclear+storage, even smaller nuclear like those SMRs/small modular reactors.

Another massive consideration with all of this is the logical location of each type of generation at the transmission level. In the event you might have to bring the grid back from 0, or even just handle expected equipment failure, the specific location in the logical grid where types of generation is attached has to consider the capabilities of each type of generation. For example in a blackout situation you can't just start a nuclear generator when the demand is effectively 0, you have to bring generation and loads online from scratch in very increments initially. During the 2003 northeast blackout there were opinion articles complaining about how the casinos were online before neighborhoods, ignorant to the fact those casinos were instrumental in providing an initial load on the transmission grid.