It doesn't seem very surprising that if you keep eating mostly what you normally eat, your microbiome is already well-adapted to that diet and will out compete

It doesn't seem very surprising that if you keep eating mostly what you normally eat, your microbiome is already well-adapted to that diet and will out compete almost anything new you try to introduce. I would expect if they tried limiting their diet to foods compatible with the probiotics, they'd get more significant results, because the existing microbiome colonies would be stressed from lack of their usual junk/meat/fat and be weaker. (Almost all standard probiotics, other than dairy varieties [yogurt etc], are veg-carb-loving.) Before refrigeration, there couldn't have been many probiotics consumed in isolated form rather than as fermented foods. Give the new bacteria some food to eat when they're trying to out-compete your existing (bad parts of your) gut microbiome. One of the most likely good foods for them to eat is what they've been cultured on. It may not even be necessary to eat probiotics, if you already have some of the bacteria you want, you merely need to encourage it to out-compete bad microbiome bacterial strains. Eat food compatible with the good bacteria. But it's such a hassle to change diets, everyone wants a pill or a powder or one serving of probiotics they can add to their diet, keep everything else the same, and have the bad bacteria / imbalance miraculously disappear. Is it any surprise that's less likely to work?