Yellow and Brown Peril.
A point to ponder: there are single Asiatic nations - China and India - which each alone contain hundreds of millions more people than all the Europeans combined worldwide.
And, despite the many defects of Chinamen, they are still an intelligent, disciplined, and technically competent people, with a grudge against the West and the White race. Indians are on average less capable, but given their large population, the bell curve ensures that they too will have many millions of highly competent individuals, and they too have an animus against the West. Both nations are, of course, nuclear-armed as well.
While no one states that this fact means Europeans have to give up their uniqueness - something which I oppose - the fact does mean that the idea that European nations and narrow ethnies can "go it alone" is absurd. And any "Darwinian processes" involving peoples will likely be between, and not within, races and civilizations.
One other note: part of the problem of this debate is one of definitions. What one person means by "ethnonationalism" and "Pan-Europeanism" is not necessarily what others mean. Thus, without precise definitions, people often talk past each other.
I have already defined pan-Europeanism as a meme that acts to promote, defend, and extend the interests of ALL people of European descent worldwide.
The definition of the type of ethnonationlaim that I oppose would be a meme that asserts that the highest level of group interest is the nation/ethnic group, with indifference or even hostility to different nations/ethnies of the same broad race and civilization.
There could be an "enlightened ethnonationalism" that I would support in which a primary interest in nation/ethnic group is also extended to include, at least secondarily, the broader race and civilization.
Pan-Europeanism proper would consider both narrow and broad interests to be of equal value.