Is America not great now?
The answer depends on who you are.
On the other hand.
Why this is, is easy to explain. Jobs have gone East, migrants have poured in and some jobs are now done by machine. Carry on with this trend and many here won't have a lot of customers to sell anything to. Even those relying on the top 5% will find it harder. As more tap into a well, that's drying up.
Those in favour of the top getting richer will say we can't do without those people. When the truth is those people can't do without the West as a market to sell in. Until the old Third World replaces that market.
Imagine the wealth of Jobs, Gates, JC Penny, Walmart etc. If their goods couldn;t be sold in the US?