Even if it takes 20 years to build a nuke, that might still be sooner than "power to x" becomes a viable, affordable option for most of the world.
This is the weird thing about this argument: it so often rests simultaneously on an argument for practicality and/or cost effectiveness, but literally any criticism is met with a gish gallop of unproven technology. You can't have this both ways.
In the end, my central thesis here is just that if you think the be all and end all of this issue is cost effectiveness you are optimizing for the wrong goals and it leaves open a giant window for carbon emitting sources to justify their use when renewables don't work for various local reasons.
The thing to be black and white about is carbon emissions. "What energy mix works for place X right now if we try to eliminate carbon emissions asap" is not a decision that's gonna be made on an internet forum.
A nuke powered world will either require radically new sources of uranium (like sea water uranium extraction) or breeder reactors. Neither is available now, unlike cheap electrolyzers for making hydrogen (< $300/W in China).
Again, this is not a duel to the death. The only thing that should be dismissed out of hand is carbon emitting fuels.
But instead of being so "black and white" and insisting it must be nuclear that powers things, also evaluate options like thermal storage to gather summertime heat in reservoirs for use throughout the winter. It's super cheap, and makes a ton of sense in most areas, yet it's so low tech that it gets ignored.
I will be black and white that fission is not a feasible solution for the vast majority of the human population for at least the next 20 years, and probably forever. Despite more than half a century of experience, it has not improved as a technology. There would have to be some sort of drastic breakthrough for nuclear to be a realistic power source for most of the earth's surface.
It sure is a good thing I didn't say anything of the sort or wow I'd be a hypocrite.
I am definitely not the person trying to shoehorn the entire earth into one energy mix box in this conversation. I think the answer to this is likely to vary a lot all over the world.
Also a lot of the world's population is far enough north that winters produce relatively little solar or wind energy. And those places also tend to be massive energy consumers because it's also very cold.
Right now that doesn't show up in electricity stats because of how common in-home gas heating is. In order to decarbonize the northern parts of the world we will need to dramatically increase both electric energy consumption and production, during winter, because heat pumps will almost certainly be replacing gas furnaces.
Frankly, whenever this comes up the handwaving about storage just seems hopelessly optimistic even for places where it is practical, let alone places where you're lucky to get 4 hours of sunlight a day for a bunch of the year.
Nothing is ever that simple. If carbon zero is an imperative, then there will sometimes be need for something other than solar and wind, and it will inevitably be "impractically expensive" compared to the carbon emitting sources we're moving away from.
It's certainly important to improve the economies of scale of renewables, but doing so is not the goal. Saving money is not the goal. The survival of the human species is the goal, and we aren't going to get there by nickel and diming our way to it.