Nations rarely go down. Sure, there are things like the fall of the soviet union, nazi germany or yugoslavia, but thats the exception.
So, the USA as a nation isnt going anywhere, and isnt doomed. However, I think claims such as this are more intended to say the "idea" of America, the America of right now, is going to go away.
Which is utterly right, as America has always been a nation of change and evolution, into pretty much any direction.
In my opinion and limited experience, people "doomsay" in times of uncertainty (obviously), but also a lot in times of a strong polarization of public discourse and society. The more pronounced a so-called rift in a populace becomes, the more people panic.
And if you mean the "American Dream", that one died out long ago. Some just never noticed
