Nuclear isn't flexible capacity; plants are always run at max output. If it is to be phased out the replacement needs to have more long-term output than a battery, unless the renewables become incredibly overprovisioned.
In France and Germany, nuclear reactors are configured and permitted to run in load following mode. It's harder on the valves and similar equipment, but it can and is done. [1]
> unless the renewables become incredibly overprovisioned.
Which is the likely outcome, as solar and wind are still declining in cost and in some places the cost is as low as 2 cents/kwh. The generation will be cheap (what's the old saying? "too cheap to meter"?), and it'll be the storage that'll cost a bit more.
[1] https://www.oecd-nea.org/nea-news/2011/29-2/nea-news-29-2-lo...