There's an engineering tradeoff between having excess generation capacity, for that seasonal minimum, and having more storage. The cost optimal decision on that tradeoff will be determined by the ratio of excess generation cost and storage cost for rarely-used storage.

Until that ratio falls by at least 3x to 5x in favor of batteries being cheaper than generation, extra generation is going to be the way that grids actually get built out. Batteries and generation are both falling in cost fairly quickly, but generation still has the overall edge in learning rate. Cost decreases won't bottom out for at least a decade, because there's been no slow down yet, so I wouldn't expect this ratio to change for a minimum of 20 years, which means that pretty much a full energy system interchange will have happened by the time that this price ration changes.

So there's at least a few assumptions about the current industry and it's future development that underlie my assertion that a month of storage makes no sense, but I'm confident enough that I'd place money on the bet, and there's very very few things I'd bet on.

Edit: one thing that would break my assumption is the industrial development of storage that's super cheap for once-per-year usage. Most storage now needs to be cycled about 300x per year to make economic sense. "Long duration" storage is actually better defined as "economical storage at few battery cycles per year". There's nothing like that in the research hopper, but that doesn't mean it couldn't appear tomorrow and be deployed within a decade. Something that only gets used once or twice a year has to be dirt cheap, even if you could get 10x or 20x normal electricity prices for it.