We know that we didn't evolve from chimps...but what if we didn't?
But we know that we didn't evolve from chimps because chimps are around today and we don't have chimp fossils from as far back as we have fossils from the non-chimp, human branch. You're creating a scenario with no chimps around today, and then judging its validity based on what we know from chimps being around today. It's self-contradictory.
Besides, from what I've read, australopithecus isn't considered an "ancestor" in the sense that there's a direct line of descent from them to us. (I called them "transitional.") Rather, they're a dead twig on the same branch we're on. If there are no bipedal apes at 6 MYA, and on-their-way-to-bipedal apes (and humanlike hands et al.) at 3 MYA, and fully bipedal apes with big brains at 1 MYA, and now us, I don't see any problem calling those other ones "transitional," even if they're not direct ancestors.
What I'm saying is that dead twig or not, it's quite possible certain forms are being called transitional forms merely because they seem to fit a certain type, not because there's evidence that they evolved from the previous form and into the next form.