By the year 2100 (last year of the 21st century), the entire world will be what we current call "developed," as in the developed world.
The rich will be richer still.
The middle class will be rich.
The (majority of the) poor will be middle class. But for political reasons will continue to call themselves the poor.
A small number of the poor still won't have figured out the system, and will be actually "poor."
If the United States still exists in 2050, it will remain in existence is 2100. If it has effectively disappeared by 2050 then what will be present in 2100 will be a balkanized land mass of pocket welfare states barely hanging on, situated right next to multiple small, wealthy, well armed super powers.
Are you really not aware that your first set of predictions is utterly inconsistent with the second?
How so? The first prediction is world wide, the second involves only the US. If we continue to exist as a nation things will be good, consistent with the world wide prediction.
If the nation dissolves, then portions will succeed, while others will be worse off, but will still be "developed" by the current meaning, but at the same time struggling in comparison to their neighbors. Think UK and Germany, verses Greece and Spain.