It's necessary (or at least standard practice) in a lot of dairy farming regions. Consuming lots of dairy is typically a high-latitude thing.
You could only have dairy farms in places that don't have a winter, but that would be pushing things even further toward the whole "put all the cattle in Californian factory farms and then burn lots and lots of oil to ship it around in trucks" thing, which is arguably even less optimal.
Or we could just resurrect older breeds descended from Aurochs that had shaggy coats. Or we could encourage the consumption of traditional dairy products in those regions like reindeer or yak-based dairies.
Factory farming in NL versus California is probably pretty similar. California's not really a great place for ranching in the first place though, better to just do that in a place like Vermont with a lot more water.
I suspect that even if you could leave your herd out in the snow all winter, a detailed analysis would determine that bringing them into the barn to keep them warmer and feeding them hay for the winter yields more milk per acre. That would, in turn, imply things like a lower carbon footprint and less damage to already stressed ecosystems.
It's kind of like burning wood to heat your house: It may be traditional, but that doesn't necessarily make it better. Realistically, the only reason all sorts of traditional practices were sustainable way back when was that the human population used to be a fraction of what it is now.
> Appeal to nature doesn’t imply that it’s the optimal thing to do.
It doesn't, but it's not meaningless either. It can be a useful heuristic, for example when trying to judge what conditions are better for the animals involved. Nature sucks in various ways, but 'unnatural' conditions that the animal is not adapted to living in often do cause more mental and physical distress.
You could only have dairy farms in places that don't have a winter, but that would be pushing things even further toward the whole "put all the cattle in Californian factory farms and then burn lots and lots of oil to ship it around in trucks" thing, which is arguably even less optimal.