> "Old nuclear" is fundamentally not cost competitive with current solar/wind,
That's like saying that someone who owns 90% of a publicly traded company is worth $9 billion just because the stock has a market cap of $10 billion. If they'd actually try to cash that stock, they'd run out of buyers and the stock price would go way, way down.
Solar and wind are great now, because we can pick a random spot and build it without needing storage and don't have many people objecting to it. (Yes, the lawsuits are there already, but so far we've only gotten to the low hanging fruit. It'll get much worse before we're anywhere near 50% green energy.) It's cheap if you just look at the cost of the pure panels/turbines.
The earth has a finite amount of habitable land. If you start filling that up with solar panels, land price will increase because you have to buy it from people that wanted to plant crops there or live there. A few countries like the USA and Argentina have it easy, but most places don't have large swathes of land laying around completely unused with wind and sun aplenty.
We currently use about 162 PWh worldwide, only 27 of which is electricity[1]. We're barely starting to make a dent in electricity production and what you hear left and right is "not in my backyard" and "not on my the neighbor's roof that I have to look at every day" (--my dad). We'd barely have the space if people wouldn't complain, let alone if we can only build it away from civilization. And that's replacing electricity production alone, we haven't really started on transportation, making things (cement, steel, plastics, ...), growing things (fertilizer, cow farts), and heating/cooling. What's more: due to only replacing a tiny percentage of electricity production with solar and wind, we aren't yet seeing the need for large scale energy storage, which seems like it will easily triple the cost with current technology (my takeaway from [2], p. 76).
You might counter with putting the stuff on sea instead to avoid the finite land issue. The primary problem there is that this is not where people live, increasing the cost of construction, maintenance, and energy transportation. For example, wind on sea is about as expensive as nuclear (I should start keeping notes on which sources I looked at before and copy-paste texts instead of memorize and reiterate). Solar I don't know, but for some reason we're not doing that on the same scale as wind, presumably for a good reason. There just aren't any easy shortcuts here, only compromises.
Let's not dismiss fission energy just yet. It might seem expensive today, but between a known ~2x price increase (iirc going full nuclear would increase the energy price by that much from today's price, though France has me wondering if I might be misremembering) and a larger cost if you do solar/wind+batteries+need-to-win-lots-of-lawsuits-for-space (which works out to be about 3x for just the hardware, using today's technology), I'll have fission reactors please. Until we have better tech, let's get started with the current tech and hope it becomes obsolete and we get cheaper energy with a sunk cost rather than not try because we might sink some cost.
[1] https://en.wikipedia.org/wiki/Primary_energy
[2] How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need - Bill Gates - 2021