Sun can be predicted. Not sure what the reaction time is and how far ahead we can reliably forecast wind, but even if there is zero response possible to wind fluctuations that still seems like a win. Afaik fuel is also not the biggest cost in nuclear so estimating on the high side might not be an issue (but this is admittedly where I veer into speculation).

Uranium (as a nuclear fuel) cost may never become prohibitive, however ore grade may prove a major challenge as its decline leads to more emissions: https://news.ycombinator.com/item?id=30660554