Leaving aside the issues with directly comparing plant milk with actual milk, the economics are not what you lay out. Oat milk is really yet another industrial corn product. The caloric content isn't cloudy stuff squeezed into water, it's fructose from corn. In the quest for price stability, corn subsidy is one of the main knobs used by USDA from a subsidy and policy standpoint.
I haven't done a deep analysis of this, but I do buy milk from a farm with a smallish herd and a direct-to-producer model that allows the farmer to actually operate his own dairy and sell milk at a profit. Even with the small scale of the operation, milk costs about $6/gallon. When you buy milk from a scaled dairy operation, the economies of scale are massive... you have 1-2 processing plants handling the milk of tens of thousands of cows.
The plant milk phenomenon is all about allowing massive agribusiness to take over a big product category that has remained regional.