Starting w/ finished product =/= "trapping what's floating around"

However, the finished products of fermentation are not a conductive environment to starting the process. Such is certainly the case with lactic fermentation where the PH and concentrations of final stage lactobacillus (or Pediococcus) colonies are antagonistic to the coliform -> Leuconostoc -> lactobacillus succession.

Fermentation is a biological war over resources. And like any war, it is won by favorable conditions and superior numbers. The question then would be: are sufficient numbers of kefir micro colonies present in the final product to overcome competition for limited resources in a new environment before another micro organism gets the upper hand, or those resources become depleted. Or stated another way, are a small # of kefir micro colonies sufficiently vigorous to overcome the competition for lactose.

I believe the simple answer is no. In my opinion Kefir is not a particularly vigorous competitor, but instead relies on large established colonies vastly overwhelming the competition through sheer numbers. luckily these colonies divide and can be traded.

Kombucha on the other hand is vigorous and aggressive, hence the repeated success culturing it.

just my $.02