I have some background in Biology and have a few observations. It's important to remember that Yeast are a type of Plant. They are not animals. Originally when I went to College there were two Kingdoms, Plants and Animals, with a Quasi Third kingdom called Protistas. Bacteria and Yeast fell under the plant kingdom.
So you are talking about a Yeast Organism in abundance in your culture, and another contaminant Yeast culture is introduced. In my opinion the contaminant will flourish if the environment is favorable to it. If the contaminant is a constant component of say, your flour charge, then it may eventually win out, all things being equal. This pre-supposes that the environment established by the original yeast is conducive for growth of the second yeast strain. The yeast consumes the proteins and carbohydrates of the flour. When I took Microbiology and Cellular Biology, a first experiment you do is to liquefy Brewer's Yeast(accharomyces cerevisiae) with warm water and watch them multiply under the microscope in real time. These "Plants" can propagate at an amazing rate!
Do I know the answer to the questions posed? No I don't, I'm just offering some Biological information here on how Yeast and Bacteria behave. Can yeast Cannibalize each other, I've never heard of that and frankly am a little skeptical about it. Can a strain of Yeast produce a byproduct of metabolism that would make the environment unfavorable to another strain, that might be possible. It would be interesting to validate a Cultures identity through lab analysis/Identification. Keep it alive for a time and Analyse/ID it every year to see if it could maintain it's identity.
Under sterile Lab conditions, it is sometimes hard to maintain pure specimens. If your specimen becomes contaminated, you can isolate a portion of it that you think is pure and keep breading and diluting till you get a pure specimen again. However, this is done under strict sterile conditions with everything being introduced as sterile also. You also use a culture medium that favors the growth of the organism you are trying to culture. Flour and water favor growth for a number of different yeasts and bacteria.
So the real question is, who thinks their flour is sterile?