Kate Yoder of Grist reports that the US Food and Drug Administration (FDA) is considering the matter of plant milk. More specifically, the US dairy industry is trying to get the agency to create regulation restricting the term "milk" to the product of lactation, e.g., cow's milk. Such a rule would ban the application of "milk" to plant-based liquids, e.g., almond milk.
As Yoder points out, the first thing that comes to mind in connection with milk for most Americans is cow's milk. Of course, there is also goat's milk, sheep's milk, etc. So, on the face of it, milk refers to a liquid produced by the mammary glands of female mammals as a kind of food.
As Yoder also points out, the term "milk" has been applied to juices derived from plants for at least 500 years. The Oxford English Dictionary gives as an example "The mylke of the figge tree" from ca. 1400.
The OED also points out a particular instance of the latter in American English, that is, coconut milk. Application of "milk" to this fluid in the US goes back to 1840. Though coconuts do not lactate, this usage seems to be well established in American usage. Speaking for myself, I would be hard pressed to think of what else to call it.
The dairy lobby argues that consumers will be confused by the application of milk to plant-based liquids such as almond milk. Consumers may assume that these fluids contain the same kinds of nutrients as cow's milk and adopt misguided diets as a result.
Critics of the proposal argue that the public has a broader understanding of "milk," as illustrated by the history of its application to coconut milk and the like. Thus, the proposal is both historically infelicitous and illustrates that confusion is not likely.
I cannot say what the FDA will do. However, I think that the matter ultimately depends on authenticity rather than diet or linguistic analysis of "milk."
As to diet, producers of almond milk, etc., have and will continue to produce plant-based liquids that substitute cow's milk in various ways, e.g., pouring into coffee, on breakfast cereal, baking etc. (Full disclosure: I cook with coconut milk, and not cow's milk, regularly.) If plant-based milks can substitute for cow's milk in this way, then there is no harm from confusion over milks to protect consumers from.
As to linguistics, I would say that "almond milk" is a more-or-less literal application of "milk," rather than being a metaphor or analogy. When Shakespeare speaks of the "milk of human kindness," everyone recognizes this expression as a metaphor. Similarly, when doctors speak of a "milk-like" or milky bodily discharge, it is clearly an analogy.
"Almond milk" is no metaphor. Neither is it an analogy precisely because almond milk is for consumption in the same manner as cow's milk, unlike a milky bodily discharge. Though almond milk is not so protypical as cow's milk, it is literally milk.
These observations suggest that the case for restriction of "milk" to cow's milk relies on authenticity rather than consumer protection. Consider legal restrictions on appelations to wines. As is well known, the term "champagne" is restricted in many countries to sparkling wine from the Champagne region of France. This restriction has nothing to do with consumer protection: It is perfectly possible to substitute sparkling wine from elsewhere without risk to one's diet. Instead, the measure is justified solely on preservation of heritage.
So, if "milk" is to be restricted to mammal milk, then I suggest that justification ultimately rests on the claim that this form of milk is the most authentic. That is, it is an appelation embedded in American culture and is, as such, worthy of protection from the depredations of producers of "almond milk."
My own feeling is that this argument is also not persuasive. As noted above, Americans invented and have long been happy with the term "coconut milk." So, it seems that there is no basis on which to restrict usage of "milk" to cow's milk in order to protect American cultural heritage.
Technological change often raises issues of authenticity, such as "fake news." Food is no exception. Consider the example of "Greek" yogurt. This sort of cultural matter can become a regulatory one where it connects with economic concerns such as the food industry. And so, the FDA now considers the matter of whether or not almond milk is fake milk.