You only need to look at cultures like in India, Taiwan and elsewhere that have large vegetarian population to see that your statement doesn’t hold water.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6540890
https://m.economictimes.com/news/politics-and-nation/malnutr...
Guess where do you get folic acid, b12, etc from?
As a side note, OTC supplements are not as effective. You need prescription supplements if anyone is considering going vegetarian.
Another factor is, the number of insects that accidentally get into the food supply provide some protein. I remember reading about how a more primitive lifestyle leads to more insects being in the food supply, which provides some of the necessary protein for humans to thrive. But I can't find the link at the moment.
1: https://www.thehindu.com/news/national/other-states/80-india...