In america and europe it's a certainty

Wishful thinking on your part. America is a little bit iffy, but I seriously doubt it would happen in Europe. Bottlenecks have occured to human populations continually throughout history and populations often rebounded dramatically, so the low birthrate thing is overstated, especially in an age of scientific advances in fertility. Whites could decline to 10 million worldwide and then increase dramatically in a couple generations, and they are still at what, over half a billion?

More than likely Europe will eventually experience remigration, especially once the globohomo Jewish empire falls, and were already seeing the beginning of the end of that. Ironically Israel could serve as justification for Europeans reclaiming Europe. Jews hadnt lived in the area as a majority in a couple millenia, and yet they just took over and ethnically cleansed the locals, so the moral case for Europeans reclaiming Europe would be a thousand times stronger.