Why did the australopithecines go extinct?

I think you encapsulated what irritates me about many scientifically-inclined individuals: an over-emphasis on terminology and semantics instead of ideas. Overall, I think this may harm science because it shifts focus to precision in communication rather than understanding in communication.

 

You see, humans have this amazing ability to convey meaning perfectly without fully spelling out every word in exact detail. If I said "cars" when referring to vehicles in general, you would understand. If I use "species" when referring to genera, you understand as well.

But it's not a simply a matter of semantics. It doesn't matter what word you use, you're still treating the australopithecines as a single species whose destiny can be controlled by single events like the eruption of super-volcanoes. You're ignoring the fact that at most times during their long history, there were multiples species of australopithecines existing all over Africa, and their very long decline doesn't seem to be due to any single event.

Your suggestion also fails to explain why the australopithecines suffered but not other species in Africa, and there's the testability thing.