Most of that is not true. There's a wiki stickied in my profile that has a page on Aging, which debunks much of those claims. The "Intro" and "FMT" pages also have many studies showing long-term benefits.

There is a bi-directional pathway where environment can affect the microbiome and vice versa. But most studies show that the symptoms stem from the changing of the gut microbiome, and restoring it via FMT can reverse symptoms. Changing environment is much more limited, which is why FMT is being heavily researched as a treatment for numerous conditions.

There are many factors that need to be figured out, such as identifying ideal donors, and methods to completely replace the existing microbiome.

Our gut microbiomes have evolved alongside us for a millenia, and get passed down generationally. We're doing significant, permanent damage via antibiotics, lack of breastfeeding, junk diets, and more https://old.reddit.com/r/collapse/comments/bat7ml/while_antibiotic_resistance_gets_all_the. And these extinctions may only be reversible with FMT from the few people who are not yet damaged.