A great civilization can have about 200 years before it starts to crap out, and we had a good run. If you’re realistic and understand it isn’t going to last forever, you can give it a whirl and you have a chance to make something amazing. The materials are still here— we just need our minds, and some focus. Oh, and that wall.
America's cultural values were founded, basically by puritans in the 17th century. Their influence shaped this country, and is still felt today in many areas (blue laws, dry counties etc..) . But puritans were a tiny, tiny minority of European society. Most people were not puritanical in England, Europe, and certainly not the rest of the word.
Once large scale immigration began, in the 19th century, and population growth exploded, it was over. A minority belief system cannot keep hold on a nation this big, even if your wall existed.