Keep this in mind:
When I was a kid in the early 1970s, I was living in upstate NY. Many areas were still rural, and they were still as they were back in the 1920s. Talking to older people, I got to experience what old American culture was, that blend of different European lineages finding common ground. Even in the smaller city I come from, that Old American culture was still there in traces.
To my view, the fall happened rapidly, with now seeing all that Old America obliterated.