The same promise was held out for Ulcerative Colitis / Crohns. Composition of the gut biome clearly plays a role in both illnesses. But the search for small molecule drugs building on this observation seems to have been fruitless. That might be because it's not the presence of a particular molecule in bacterial poop that's influencing the disease but something about the biome as a whole. More positively, it might also be that the tools available at the start of that search were nothing like as good as those we have now as interest in the brain / biome link grows. I tend to think it unlikely that small molecules for effective treatment of anything much will result from this but I've no doubt a great deal of useful and important things will be learned in the process. I'd love to be wrong.