As they should. Ask Native Americans how well "illegal" immigration worked out for them.
I wonder if there were leftist Native Americans back in the day repeating the same lines we hear today:
"Diversity is strength! Bringing in more White men will only make us stronger!"
"Look at all those doctors and lawyers!"
"It'll be great! French food wagons on every corner!"
Many of the durable political geographies in the continental United States and Canada seemed to have collapsed (physically and figuratively) as a consequence of introduced diseases before colonizers had a chance to co-opt them, or even recognize their existence. AFAIU, this is partly why American (U.S.) culture developed a narrative of Native Americans as wild and free, savage and innocent. (Though we tend to emphasize the latter--free and innocent--these days.) They absolutely did co-opt political and cultural affinities, though, just as colonizers did elsewhere in the Americas (e.g. Mayans vs Aztecs).
By contrast Spanish colonization heavily emphasized integration and assimilation of native societies into the empire. Spanish colonization and Catholicization were two sides of the same coin. This had the effect of directly and indirectly preserving much of the culture, at least of those nations which were assimilated.
I'm less familiar with Portuguese colonization, but I'm guessing it proceeded more like English colonization. That's probably why the vast majority of slaves were brought to the Americas by the English/Americans and the Portuguese. This is true in absolute terms (about 3.5 million and 6 million respectively, out of a total of 12 million, AFAIU), but particularly in relative terms--the Spanish brought over only about 1 million slaves despite having massively more territory. (For the relatively tiny amount of land cultivated by slaves, France brought over 1 million.)
Walls are nothing new to human history - the first parts of the Great Wall of China were built ~2600 years ago.
I'm in no way an expert on history, but it seems that walled city states developed in many areas independently. People seem to generalize or abstract ideas, so if you have a city state, why not think bigger and have a nation state? From what I understand, again no expert, modern nation state borders are something that came from Europe [2], but a Sumerian king was the first to use a wall to mark a "national (rather than private) boundary"[1].
Walling off areas is in no way a uniquely European concept.
https://acoup.blog/2021/10/29/collections-fortification-part...