Several other points:
1. The OP was remarking that they had a hard time maintaining weight. Anybody who has done GOMAD (gallon of milk a day) can tell you that large quantities of milk will head off the possibility of weight loss.
2. Is there any reason to believe that the agricultural inputs to something like Soylent aren't just as heavily subsidized as you claim milk to be?
Milk is in our diet for several thousand years, not millions.
1. There's also a lot of people who ate balanced meals and kept weight. Drinking that much milk has some unwanted stuff in it - like IGF-I - which can cause cancer in those huge amounts.
2. Maybe soy, but that's a side-effect of the dairy that uses it not for the sake of human consumption. But it's highly likely the soy used is the one for human consumption, which would remove the necessary subsidies.
It is also a very inferior product made with zero genetic engineering. Just silly crossbreeding until the cow has big enough milk gains.
Don't see how it's, from a nutritional or health perspective, superior to Soylent.
Milk is also super cheap because it's heavily subsidized through taxes. I'm not saving idiot entrepreneurs by buying their unsustainable products.