America has changed.
In many ways, America has changed for the worse in recent decades.
We are told by liberals that America has changed for the better, what with diversity of immigration, homosexual marriage, abortion rights, and other special rights for grievance groups.
Even if, for sake of argument, you want to say that homosexual marriage advances civil rights, we still have seen so many changes in society which have not been positive for anyone.
We are just in that stage of denial where we have a hard time accepting it.
I'm sure that 4th century Romans didn't know for certain that Rome had fallen. The buildings were still there, people still went through the motions of daily life.
They may have known that things weren't quite the same but it's only from our perspective that we know that Rome fell.
In a hundred years people will remark on the similarities between the fall of Rome and the fall of America. The reign of the alien Barack Obama will be seen as very obvious evidence of that fall, like a barbarian ruling as Caesar.