in order to replace them we have to get them from somewhere, into the GI tract. there are literally thousands of strains in the large intestine, whereas most kefirs or probiotic will have 6, 10, maybe 12 strains. so probiotics can give us massive numbers of about 12 strains, but only those 12. but hey, it's better than none; we just have to keep adding them and not losing them. avoiding Antibiotics and making the terrain hospitable w/ prebiotics.
"The digestive tract is in a constant state of remodeling with an estimated 100 trillion cells made up of as many as 7000+ species and unknown strains. Most of these are still unknown and not all can be cultured outside of the body, leaving us with the several known species and strains that are commonly found in available probiotics."