As far as the soy debate goes, my MIL used to be a hardcore vegan for 20 years and as such her diet depended heavily on soy proteins (soy meat, soy milk, soy ice-cream, soy everything). In the past year she has reverted back to eating fish, dairy products and eggs to reduce her dependency on soy protein after reading that eating too much soy could have adverse effects on her health (can't remember the details). So the "science" must have been compelling enough to make her change her diet after 20 years. Eating a few soy beans here and there is probably just fine though .
No, all this shows is something well known to cognitive scientists: it is often much harder to dislodge a meme complex from a human host than a single meme. If we oversimplify the nutrition-related meme complex to just two memes:
1) It is necessary for humans to consume meat & dairy products to enjoy optimal health, and
2) Humans who consume a sufficiently varied plant-based diet (and sufficient calories) are nevertheless at risk for protein deficiency.
Your MIL rejected the first meme, but not the second. Her failure to reject the second meme led her to re-accept the first meme. This is a textbook example of a meme complex in action.
Thomas Jefferson famously predicted that within 50 years of his time of writing everyone in the U.S. would be Unitarian. He failed to properly gauge how difficult it is to dislodge religious meme complexes from the minds of humans - there are so many memes that need to be rejected at once that many people are simply incapable of making the leap.