I don't know who, in his sane mind, can claim there will never be periods of time with no sun and no wind at the same time. https://notrickszone.com/2022/12/07/plunging-towards-darkness-germany-sees-week-long-wind-sun-lull-as-energy-supply-dwindles/

You need a pilotable generator matching renewables. You can't do without it. The only question is how much of it you need to plan. Existing approaches are storage: batteries, hydro where it's possible (you pump the water up a dam to store back energy) and backup generators: coal, gas, and in some future plans, hydrogen.

None of these is a perfect solution (well, nothing is a perfect solution).

• Hydro: that's the ideal, but obviously, you need a very large body of water, and heavy construction. But it ends up being a very clean energy with long lifetime.
• Batteries: lifetime considerably reduced, requires very large amount of precious minerals (today, car industry assume they'll get ~100% of lithium extracted, aeronautic assumes they'll get as much as they need without counting, and then you have the energy sector counting on very large quantities as well ; there won't be enough we can extract for everyone, and lithium mines are all but clean).
• Backup generators: no need to comment on fossil fuel, but hydrogen has a big issue: it is very inefficient, ~30%. So if you need it 10% of the time, you need to plan 30% more capacity of renewable, and that's assuming you can pilot it all the way from total shutdown to 100% capacity, probably very optimistic. You will need to have it running at some minimum levels, that's even more renewables you need to keep it running.

It is not completely true that nuclear needs to run at fixed level. Depending on their design, some plants are pilotable and some are not. But I don't think (I'll be happily corrected if needed) any had the flexibility you need to be used with renewable (quick large variations).

So the ideal mix is, IMHO, a baseline provided by nuclear, and a mix of renewable and complements to produce the difference.

Bonus: there is a "method" promoted by some (ignorant) politics they call "proliferation" ("foisonnement", not sure I'm translating that the best). This is utter BS...

The idea is there will always be sun or wind somewhere in a super-grid spreading through Europe. If you think about it for 1 minute, that means that small part of Europe where there is wind will power, for a more or less short time, a large portion of the whole Europe?? Not only is that totally insane from the capacity point of view, but it also completely neglects the grid's stability and electricity transportation issue. It is very difficult to transport electricity over very large distances without disturbing the grid. Ask Germany, they spend massively on infrastructures right now without counting on proliferation. That would raise the requirements further...