Agree OP. Gut health isn't a fad diet. Poor gut health really is the root of the declining health in the western world, including mental health. It's not just down to diet, but clearly that's a huge factor.
of course there will be people just jumping on the bandwagon that don't really know what they are talking about. I include Tim Spector in this. He probably believes what he says, but I honestly think he's massively behind the curve, pushes a vegan agenda (dreadful for good health) and I've just learned Zoe recommends rapeseed oil 🤦♀️
Seeds oils are literally poison to the human body, (excess of omega 6, not enough omega 3), hence the major decline in health since their introduction.
The latest 'fad' diet is carnivore/animal based. Look at Paul Saladino MD. However it actually makes a lot of sense, follows an ancestral diet (well raised meat/eggs/fruit/raw dairy/ raw honey). This guy hates vegetables 😂 (fermented are better of course). he is a little extreme. But I think the basics are there.
See also Weston A Price foundation, and Lily Nichols RDN. Dr Nemechek 's gut health protocol.
Although there are differences of opinion, for me, there is enough agreement on certain principles ( eliminating seed oils, reducing carbs, benefits of good olive oil, benefits of animal based foods, etc) to know it isn't just a fad. The vegan movement is not great for human health. But if done without seed oils then still better than a standard modern diet.