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.