It is interesting that they achieved any positive results with prebiotics. The animal study evidence suggests that these are weak interventions compared to fecal microbiota transplant (FMT) from a young individual or flagellin immunization. Weak in the sense that they don't last long for one dose, whereas both FMT and flagellin immunization are essentially years-long to permanent effects, and also weak in the sense of a small effect size on the gut microbiome and health measures compared to FMT and flagellin immunization.

In principle it should be possible to produce some sort of one-time high dose oral probiotic that produces results that are in the same ballpark for effect size and duration as FMT. In practice, it doesn't look like anyone is rushing to get that done, and it might turn out to be very expensive to manufacture 100-200+ distinct microbial populations into a probiotic mix. No-one seems to know whether it would take that many, or whether there are a lynchpin few dozen species one could focus on to get an 80/20 outcome. But that sounds like a question that could be answered in the next decade or two at the present pace.