I can't comment on per capita consumption in 1980, but this data[1] seems to suggest that Americans are actually consuming _more_ dairy products per capita than in the year 2000 – 643 pounds per year in 2017 vs 591 pounds in 2000.
From 2017: oversupply and plummeting prices https://www.marketwatch.com/story/got-milk-too-much-of-it-sa... ...in Ag, it’s not always a good thing to be over productive especially if there are subsidies in place. Price controls and free market throttling will always be suicidal. Not just for dairy..also true for other commodity crops.
[1] https://www.ers.usda.gov/data-products/dairy-data/