I do not know that assimilation for today's immigrants to western countries is possible. But I will say that assimilation is a bare minimum requirement for any immigrant coming to the US or any other western country. By moving here, you are admitting that we are doing something better than your country of origin. And if we are not, then why are you here instead of there?
Being here for economic reasons alone shows you are not assimilating, and you therefore should not be here.
On this topic, I see why libertarians have no issue with immigrants solely here for economic reasons. Because they seem to only view countries as economic zones themselves, so they see no issue with immigrants viewing the country as that same way. But they fail to realize that immigrants don't share the same view on general freedom that they do. They simply demand more laws and policies in their favor while seeking the most economic advantage they can.
A lot of libertarians fail to understand that when an immigrant comes from a country with a failing government system, most of them will not come to the conclusion that government itself is the problem, but rather that their government failed. They look at countries like the USA with a relatively "better government" system and see latching themselves to that system will improve their situation. They are not necessarily incorrect, but this creates further issues.
Another form of necessary assimilation is conforming to the citizens ideas of government role and size. Immigrants that demand more of a "better government" than their country of origin are showing that they are failing to assimilate, and therefore should not be here. But those who are seeking to increase government power see opening the borders for more immigrants will take advantage of these consequences. All while traitorously claiming that kind of immigration is "an American value."