If I was a nationalist in any country with roughly the demographic shape of Germany (so, most of the wealthy ones), I would regard open immigration as the highest possible leverage move, to the point, where I (not, in fact, being a nationalist) actually feel bad about the devastating effects this is ultimately going to have a few years down the line for the countries these people are leaving behind.
Bar an AI miracle, everyone is going to need bodies, and the bodies they are going to produce, to avoid societal breakdown. By current looks, it is potentially the scarcest resource of the future.
I understand that immigration of unskilled people is a little off from the "skilled immigration" pipe dream and less convenient, but are people really that confused about the slightly longer term and exponentially scarier cost of no immigration?
These countries have a system that, while not perfect, is the most excellent at creating skilled enough humans. Letting anyone willing and able to produce a child in the country, that you can then pump through that system, is the absolute cheapest and most realistic way you are ever going to get the amount of bodies you are going to need to avoid a total demographic disaster.