Soybeans are a fairly large part of most purely vegetarian diets, as they provide a metric ton of the various nutrients one needs, you can get around that with enough other types of vegetables but SB is the quickest and easiest which means it would likely become one of the most heavily consumed parts of an all vegetarian economy if everyone were to suddenly switch.

Soybeans, kale, and winter squashes are generally the most popular because they are very heavily nutrient and vitamin dense. But they don't exactly scale to extreme levels cleanly.

Again, to be clear, I am advocating for a switch to primarily vegetarian diets. I'm just saying going exclusively vegetarian isn't necessarily going to be great and will come with its own set of challenges and problems. A middle ground of primarily vegetarian but still some light meat usage is going to be the healthier Middle Ground both for your diet and the environment