Creating an underclass is a terrible idea. So we should strive to ensure an egalitarian, pluralistic society in which people can live alongside each other with respect.

(This includes not making an underclass out of people who are already citizens, either!)

This is the correct goal. But work on reducing the existing underclass is frequently undercut by importing another. Seeking not to make the problem larger, and assessing realistically how fast your solutions are working, seems like a rational course of action.

For example, the best thing that could happen to the US underclass is for unskilled labour to become expensive. If slaughterhouses and gardening firms and nail salons had to all bid for the same supply of local labour, they would bid the price up. But instead these industries are heavily reliant on imported labour. (Their owners will speak of a shortage but, as HN well knows when it's programming jobs, they mean a shortage at the price and docility to which they are accustomed.)

>Can you really ... think that ... creating an underclass with ... different phenotypic traits ... will go well?

What are you saying, exactly? Don't dress it up with $20 words... you're literally saying "Those Others are different from us. Ick!"

You're hearing that. But "look at human history" could mean thinking a little bit about the great multicultural empires of the recent past... like you know, the Ottomans, or the Austrians, or the Indian Raj.

All of these ended in great violence, and once the killing was over, a much higher degree of segregation. (Arguably what's going on in Syria is still the endgame of Ottomans disintegration.) Not wishing to repeat this historical experience does not seem crazy to me.

Do you have other historical examples? Or you prefer to just imagine sunshine and rainbows forever?

The instinct that you describe is real. You can either acknowledge it as a constraint and design your immigration policy to work around it, or you can ignore it. But you do so at your peril.