Syonyk, you have mentioned that history gives a guide that our decline is inevitable. You may be right, but my argument is that history's real lesson is things get better.
This is a very interesting thread. I'm not worried about a decline, for several reasons:
Reason #1: A middling country in 50 years will likely be a far more badass place to live than the US is today. Take the Roman Empire. Your standard of living is unimaginably better in today's Italy as a lower-middle class person than it was for a super rich dude at the height of the Roman Empire. Undeniably. Some random Italian today has better access to health care, education, electricity, communication, travel, etc, etc, etc. Do you think Italians are pining for the their country's lifestyle in the year 100AD back when they were the greatest empire on earth? They had aquaducts...cool story Romans...today's poor italians can fly on airplanes and skype anyone in the world and bank in more badass ways than the roman emperor.
If the US moves to #10 on world's richest list when I die decades from now (which I think is shortchanging the US big time), then we are still going to be living in a far better place than we are now.
Reason #2(although very related to #1): Technology. Like Boarder and others have said on here, advances to clean energy and AI will transform the world we live in. Things like employment rates will have a new meaning decades from now, because labor won't even be a bottleneck for production. The vast majority of things that today take tons of labor to do, will require a tiny fraction of people decades from now. For example, today a people drive around all day delivering mail. That will be seen as a huge waste of time in the future. How big of a stretch is it to imagine a self driving car pulling into a neighborhood with 20 drones that each fly back and forth a few times delivering the mail to each persons home, then returning to the car as it drives itself to the subdivision. Heck, I'm a math teacher. Maybe virtual reality will make math teachers as we know them obsolete in the future. In this world, stuff will become really cheap, because things require so little labor. It will free people up to do even cooler stuff, like leisure, learning, the arts.
In conclusion: The best dooms day prepping we can do is to help support policies that encourage science, protect our environment and climate, help ensure education and access to family planning to women around the world, and support poor nations to help them join the party, and implement things like universal income when it becomes necessary. The important thing isn't going to be where the USA ranks 30 or 50 years from now. The important question is can we all work together as a world to make sure this planet stays a kickass place to live for everyone. Because if we do that, abundance will become inevitable. A middle class Camboidan 50 years from now will look back at doctors/engineers/teachers from the USA in 2017 and say "jeez I'm glad I don't have to live that lifestyle".