Nazi Europe would have been subject to decay like every other society. It might have taken a different form, but they were unlikely to remain dynamic and advancing for 3-4 generations.
And would a slave society, like Hitler’s empire, really have opposed importing foreign labor, so long as it was kept under tight control?
I don’t think there would have been any Third World immigration allowed, true.
Still, there would surely be, as Mr. X implied, be other issues- maybe just as deleterious as the Dark invasion…
We simply can’t know exactly.
The very best thing might have been reforms after the demise of the first generation leaders as Hitler or Mussolini and their ilk.
After all, there were reforms in the USSR after Stalin was gone, just as there were reforms in China after the departure of the Great Helmsman, or in Spain and Portugal after Franco and Salazar had passed.
But what would have happened in the years until then?