I doubt that the cordon sanitaire ever was a useful tactic, but even assuming it was at some point, it's clear that it has stopped being useful a long time ago and the sooner it's abandoned the better. The main result of that strategy has been to make the topic of immigration toxic for mainstream parties for a long time, until opposition to immigration reached such a high level in public opinion that even mainstream parties had to shift in a restrictionist direction at least rhetorically. The problem is that, by the time this happened, support for anti-immigration parties had reached such a high level and the credibility of mainstream parties on the topic had reached such a low level, not to mention that the problems caused by immigration themselves had become much harder to address, that it didn't matter. The result is that, if you still insist on maintaining a cordon sanitaire, not only will it prevent support for populist parties from falling due to incumbent penalty as noted below, but it will increasingly make European countries ungovernable, because effectively neutralizing ~1/3 of the electorate makes it difficult for mainstream parties to form a governing coalition. But even when the cordon sanitaire is abandoned, which has already happened in some places and will inevitably happen everywhere sooner or later, it won't solve the problem because again decades of neglect have made it much harder to address the underlying issues at this point. What this means is that, because of the situation that was created in large part by this idiotic strategy and the insistence on refusing to address people's concerns about immigration, we'll be stuck with significant populist movements for a very long time, with very bad consequences in terms of policy across the board. This alone would make utilitarian-style arguments for immigration stupid even if the people who make this kind of argument weren't clueless about the effects of immigration in Europe, but they are in fact totally clueless on the topic. They like to make fun of the proles for having crazy beliefs about immigration, but in many cases their beliefs about it are at least as stupid. For instance, it's perfectly obvious based on data on perpetrators that low-skill immigrants from Africa and the Middle East increase crime in Europe other things being equal, but I constantly run into economists and other very intelligent people who cite some random IV-study that failed to find any effect of immigration on crime, not realizing that 1) this kind of study usually has a statistical power of approximately zero and 2) even if that were not the case what they measure is an equilibrium effect that isn't actually what people care about. Everything is like this on the topic. Like you can read about the magical beliefs that many wonks have about the alleged disinflationary effects of immigration and it's a similar level of stupidity. Once again, when future historians look back on this period, they will probably think that someone tampered with their sources because they will refuse to believe that anyone can be that stupid.