This topic has been hotly debated here. For example:
The deal comes down to this: if you have a healthy starter culture with a population in the billions, the doubling rate of any local interloper would have to be so much greater than the doubling rate of the starter culture that it is mathematically impossible in my lifetime for the locals to come close to taking over. This assumes a well-fed, happy, healthy starter culture. The cultures from sourdo.com are in many cases hundreds of years old that are highly resistant to local contamination.
I certainly don't want to get into a debate on what others experience and believe. I have debated with people on this topic for almost 20 years and my experiences are what form my opinion on this topic. I don't want to get into I'm right, your wrong, etc. That being said, here is an example of my experience -
Years ago, a few colleagues and I exchanged starters that we had all created from scratch in our bakeries. I was in Oregon, they were in San Francisco and Santa Cruz, CA. We all fed each others starters for about 6 months and then the day before I flew down, we all made bread from each other's starters, along with our own starter. Same formula, same flour, etc. We discovered that each persons bread tasted exactly alike. All the bread made in my bakery with all 3 different starters tasted the same, all three from SF tasted the same, etc.
I think there are too many variances for a culture to not change. Flour used has different wild yeast strains, your local has different yeast strains in the air and the bacteria available from person to person and local to local are all different.
Just my 2 cents from experience and what fellow colleagues have also noticed. Maybe baking hundreds of loafs everyday makes more of a difference. Dunno. I have baked maybe 20 loaves of bread at home, but i've made hundreds of thousand, if not millions, in bakeries.