I don't see how this follows. I've definitely been in biology classes where we discussed different groups of rats in this way and we never considered them separate species just because they were different groups.
In the long run, all nations are developed nations, and then what will we use to draw distinctions between human beings? I’m sure folks will think of something.
It just goes to show.
I prefer to just shorthand this as "OECD" versus "non-OECD" which manages to just about cover all the states everyone actually means when they use "Western" (including tons of countries that aren't Western at all) or "developed", though the latter's a decent enough term IMO. "Western" is especially silly, of course, for a bunch of reasons, including that pretty much all the "original" Western states were themselves at some point "Westernized"—rolled over by Generic European-ish Capitalist "Culture", erasing most of what was there before, and absorbing and exporting those elements that could be sold to others (those parts becoming "Western")
This is just a nitpick as I agree with your overall point, but South Korea isn't agreed by everyone to be a developed country. In fact, they are right on the cusp as evidenced by the fact that some investment funds consider them developing and some consider them developed.
I almost think there should be a classification for the US, Japan, and the UK, and another for "everyone else". If you look at the number of medium to smallish companies that have access to public stock markets, there is a huge difference between those three countries and everywhere else. Something is fundamentally different about their economies. And I've never read where someone declares this a mystery worthy of investigation and tries to explain it.
Apart from that, if you speak of group A going extinct and group B not, you are implicitly defining them as separate species. I don't think that's an accurate description of the present nor a plausible description of the future.