I agree, also at work I was thinking about the cultural aspects of this. Many of these places were traditional animist worship that saw animals as neither good nor evil. The Navajo had that change when they started raising sheep and goats. The wolf an evil force in their culture because of that. I think because hunting and warfare would still happen we would see the development of warrior type religions that worshiped Bears, Wolves and Mountain Lions, while outside of the more male dominated aspects of life like hunting and warfare they would be demonized as threats to livestock and as it was these animals were seen as a more scary animal due to the forests that they lived in leading to surprises so now that besides their own lives and family and tribe members they would worry about they would have much more invested in other animals.I'd say that's tricky. As I've argued, committing to one particular lifestyle will bring opportunity costs. A herdsmen culture has to stay near its animals, and its animals habitats, which can impact accessing marine mammals or fish spawning runs. You might end up with an either permanent or transient cultural subgroup. I believe, for instance, that the Italmen or Koryuk of the Kamchatka peninsula tended to partition themselves as Seal harvesters or Reindeer herders.
Reindeer are milked, but they're not particularly bountiful for milking. Of course, selective breeding over an extended period of time may change that. I guess the trick is to determine how much this particular trait may induce selection. Also, while the lactose tolerance mutation does seem to occur among the inuit, I don't know that it's prevalent in athapaskan populations.
I think the most significant potential for milk might be in reducing infant/juvenile mortality.
Hunter Gatherer populations are always low compared to the environment. Usually about 25% of what the environment seems able to support, often much less. This is because Hunter Gatherer populations are surviving on what the environment gives them at any particular moment. This means that populations are not governed by the best month, or even the average month, but by the worst months. The year round population is the population that managed to avoid starving or dying in the hungriest, leanest season, during the drought, the year the caribou didn't come, the bad winter that you couldn't catch seals.
The most vulnerable to this bad season bottleneck are infants and juveniles, the most rapidly growing, the hungriest, the least valuable members of the community. Seasonal malnutrition will kill some, and stunt the growth of others. Give this cohort a food source to help tide them through, you'll get more and healthier survivors.