"I have always thought it rather sad that the isthmus arose, and led eventually to the extinction of all that "parallel" marsupial fauna."

I agree. However, it also had benefits elsewhere, as I pointed out.

Without the isthmus, all those weird, giant species might have endured (I say "might" because, of course, there was a broad extinction event some 10K years ago that wiped out most of North America's giant fauna as well as South America's. So the giant saber-toothed opossums might not be with us today, even without the isthmus.