But in what way does that change the meaning of the established linguistics? That's the part I'm struggling to grasp. I understand the commercial milk producers wanting to muddy the waters from a competitive perspective, but why should you or I want almond milk, or other plant based milks, called something not 'milk'?

Unless you're drinking milk from the cow's tit, your milk is very mucg an industrial product to make it shelf stable and consistent. People have a totally wrong idea of what real milk feels or tastes like or what's involved in its production. At least oat milk is literally just filtered porridge you can make at home.