Just for the record, that first image is a generation of normal curves of arbitrary and identical variance. In reality, the ethnicities also differ in variance.

I couldn't find an image that illustrated both the difference in the curves, as well as the difference in variance for the races, do you have one?

It's not being discarded.

It's being forcibly excluded from the local economy, that is at least by some measure "discarded".

What's being said is that these ethnicities operate more efficiently in their cultural centers.

Then why was Apartheid South Africa and the Belgian Congo so infinitely superior to the present state thereof in both cases, if ethnicities operate more efficiently purely in their cultural centers? Why was post Japanese / Western intervention Korea so enormously more developed than its insular ethnic enclave status beforehand? Why was Japan infinitely more powerful after the Meiji restoration compared to its prior insular shogunate? What about the effects of European colonisation on North America, Australia, New Zealand, South America, Canada... The list goes on.

You can still have trade and diplomacy between these ethno-nations, and such trade would be conducted more efficiently than if you forced them to intermix.

That's a false dichotomy, nobody is claiming that any ethnicities ought to be forced to intermix, merely that they should not be forced not to. Also, you haven't at all objectively proven that trade between ethnostates is "more efficient" than trade between free societies.