My mom, who gave up eating meat at age 85, has often talked about their diet when she was growing up, and how infrequently and small amount of meat they ate compared to the 'typical' north american diet.
But I don't really agree with the article - they are basically saying that a vegetarian
diet is not incomplete because it includes a variety of different protein sources. That doesn't change the fact that individual vegetable proteins are 'incomplete' and that we are complementing them by eating a diversity of fruits veggies and etc.
It's also true that vegetable sources of protein are somewhat less bioavailable, some contain antinutrients as well, so may need to consume somewhat more of them to fulfill your protein requirements. Not saying that it can't be done, but these are the facts.
I don't like that the article trivializes protein requirements, which vary quite a bit depending on your lifestyle and your age. If you're a young journalist with lifestyle behind a desk, you don't have the same needs as a farmer, an athlete, or an elderly person. There are specific benefits from animal proteins (as there are from vegetable proteins) which may be less important or more so, depending on the individual, lifestyle, age, health conditions, and probably genetics.
This article that reviews protein types including supplements for athletes also looks at some of the issues commented above.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3905294/
This is what they say about the "complete vs incomplete" question:
"Typically, all dietary animal protein sources are considered to be complete proteins. That is, a protein that contains all of the essential amino acids. Proteins from vegetable sources are incomplete in that they are generally lacking one or two essential amino acids. Thus, someone who desires to get their protein from vegetable sources (i.e. vegetarian) will need to consume a variety of vegetables, fruits, grains, and legumes to ensure consumption of all essential amino acids. As such, individuals are able to achieve necessary protein requirements without consuming beef, poultry, or dairy."
There's absolutely no difference in the conclusions here, but they don't use doublespeak to throw out the established terminology and facts.
Don't forget that the same research on food nutrients is used to design and produce bulk diets for animals. You can't just feed the chickens barley. Or any one grain or legume. And that is because, individual vegetable protein sources are 'incomplete'. So I don't see that anything is gained by changing these factual terms. They will continue to be used in science, for sure.