The biomes are what it says - not one species, not a few of them, whole fucking biomes! There are thousands and thousands species, bacterial worlds inside of us, and the species interact in complicated ways. It’s not just the presence or absence of a species that is beneficial, it’s often a question of combination of species that makes a huge difference. As was said, we’re just beginning to understand. But I’d bet any treatment that makes the system simple will have some serious downsides that will appear later, when we know better.