Disagree on many, many points.
First, Germany. Immigrant minorities gravitate more to the SPD than Merkel’s CDU, so if Merkel’s trying to pad her party’s demographics through immigration, that would be a stupid strategy. Also, it’s much harder to become a citizen in Germany than the US, so many immigrants, whose families have lived in Germany for generations, cannot vote. No birthright citizenship. Immigrants have even less electoral influence there than the US.
“Open borders” is a heavily abused term these days, so I don’t blame you at all for this. Nevertheless, the only open borders the US has are between one state and another state (same in Germany). The international borders are absolutely not open, nor is there any significant group advocating for that in either country.
The US has quite a lot of undocumented immigrants, but at least lately it’s a fairly static amount. The net amount of undocumented immigration into and out of the US for the past decade has been approximately zero–with possibly slightly more leaving than arriving. So the presence of undocumented immigrants is actually a very different issue than border security.
I think “social consequences” needs a better definition. The economic consequences of immigration has been well-studied and there really isn’t much one way or the other. Immigrant communities (documented, undocumented, refugee, etc) do have a lower crime rate than the general population, but I don’t think the numbers are such that they lower the crime rate of the country they live in by any significant amount.
Now I’d agree on some points–the coalitions are shifting, but not as much as they appear to be. Even back before unions had their backs broken, the upper midwest white working class was a solidly Republican demographic, the only question being how many votes the Dems managed to peel off. Reagan Democrats and Trump voters–in that demographic, it’s the same thing really. I’d like to say they loved Dems for strong union support/good jobs and abandoned them over free trade/job losses or somesuch, but AFAICT the evidence doesn’t really support that narrative. They mobilized against welfare queens in the 80’s, and bad hombres today. Lee Atwater was absolutely correct in his assessment of the electorate, and it’s still true today.