We are called mammals because we nurse as young.
Only humans and their domesticated species have made milk drinking a way of life past infancy.
Can you name a single wild mammal that drinks milk?
The question to you was not how something was going to get something else. The question was whether something had a use(function) or not? I really don't care whether something avails itself of the use of something, since the question was whether that something had a use or not. A tiger or raccoon or rat can get milk from a teat, whether they choose to or not. And we are called mammals due to the mammary glands possessed by the class. The point being that mammals by default possess the ability to utilize lactose. The just-so story about advantage "How about mutations in the proximal region to the lactase gene that allows people to continue drinking milk into adulthood? Not an advantage?". Well since I don't have a herd of cows, and I enjoy milk and ice cream, I don't quite think I need an "accidental" mutation to continue drinking milk. You yourself have stated that as fact, "Only humans and their domesticated species have made milk drinking a way of life past infancy."
What I do consider as an explanation for difference in lactose metabolism by certain individuals is something along the lines of this analysis within a paper by Dr. James Shapiro.
A 21st Century View of evolution, James A. Shapiro
The importance of the organization of the various lac regulatory sites is that they permit the molecular computations that allow E. coli to discriminate glucose from lactose � that is, to control expression of the lactose metabolic proteins so that they are only synthesized once glucose is no longer available. The basic biochemical reactions and molecular interactions involved in this computation can be stated as logical propositions that can then be combined into partial computations (Table III). These partial computations illustrate the molecular logic allowing the cell to execute the following overall computation: "IF lactose present AND glucose not present AND cell can synthesize active LacZ and LacY, THEN transcribe lacZYA from lacP."