Some parts of it are... there is a certain amount of either nuclear base load or storage infrastructure needed.
But more than 50% renewables are easily doable without much strain on modern grids (modern models for nuclear usually plan with ~30-35% minimal base load needed...).
Also another argument for specific fossil fuels -domestic availability and thus more independence- is also a pro argument for renewables.
So no matter where you stand, if you are not at least using (or planning to use in near future) 50% cheap renewables in your electricity mix (or see narratives about expensive renewables even) then you know that the decision is not taken for economic reasons but because decision makers are getting money to stick with more fossil fuels than necessary.
Renewables are an economical no-brainer right now. Actual complexity of balancing out their fluctuating production with base load or storage comes much later (although in the case of nuclear the build times are often long enough that you should start soon). And still there are so many countries so far away from the absolute no-brainer amount of renewables.