Guess the main point is the environment itself changes depending on what you do and what you eat. So you need to constantly resupply the initial good bacteria for them to keep holding on
Is there any reason you can't just propogate and cultivate them in a seperate breeding receptacle for an unlimited supply like people do with SCOBY or whatever for making sourdough and kombucha?
Then unless your mouth ends up bacteria-free, the good bacteria are by definition not out-competing the bad ones.