I do not disagree at all that diversity is good, even God-pleasing, and it's a good thing to preserve distinct cultures. American culture is a distinct thing, as is African American culture. These are worthy of preservation. What we shouldn't have is a bunch of other foreign nationals coming over and maintaining their own cultural mores in our nation (e.g., Chinese, Mexican, Arab, etc.). Either assimilate into our culture, or stay in your own cultural lands.
And, yes, this has happened historically. You can see cultural assilimilation in the Bible for goodness sake. If cultural assimilation did not happen, we'd all still be Assyrians or Babylonians or something.
The tone of your response now seems different to those of the posts I replied to. Perhaps I didn't see your point then. Anyway:
Why bring in these people, and thus have to ask the question of assimilation at all? Why must the burden of an influx of peoples be placed on only certain countries? This isn't really a question to you, given both neo-liberals and conservatives are intent on injecting the unnecessary genetic diversity you seemed to praise earlier. But I think the difference in our opinions towards this is more a matter of degree than kind, except in the matter of the fact that I look at people as belonging to different "stocks". I don't say this to make comparisons between stocks, just to note that they seem inherently different. A Somalian Japan may not be the Japan we have today.
That being said, I think you're over-optimistic about the feasibility or goodness of assimilation, given the "countless" (to borrow your language) counterexamples to your point that also exist, including the one we empirically see playing out in front of us
today. Cultural enrichment seems to replace its host with a glob of paper citizen urbanite consumerism.
About the slavery thing: yea, your point stands that it is incorrect to assume no black people would exist in the US if the constitution was written differently. I suspect radically fewer would be, but I can't be bothered to back that up. Still, I think your point about a lot of the anger with African-American culture originating from their poor treatment also stands. Why so much of the identity seems to be focused on this is a different discussion. I was also being particular to be an ass.
I need to go outside a bit after this virus blows over. I'm starting to sound like a basement-dweller, although the opinions themselves I hold.