the billionaires are lowering the interest bill as they compete to purchase treasuries. (the money is borrowed from whoever will accept the lowest interest for it. see https://www.treasurydirect.gov/instit/auctfund/work/work.htm under "Bidding".)
The only country in the world that just prints money when they need it is USA. No other country can do it — it'll just cause hyperinflation. China is not an exception. They can't print US dollars. And if they would print more Yuan, it doesn't mean that China billionaires would be able to buy more houses in LA - the exchange rate of Yuan to dollar will simply drop and they will be able to buy exactly the same amount of goods in US as before.
USA can print almost unlimited amount of money because they control almost all the world and bend most of the governments to buy US treasuries. This is why there're US military bases in many countries and if some country decide to stop accepting US dollar for foreign trade, the next day it's invaded by US military.
Although many people predict that this uncontrollable printing of US dollar will cause US economy to collapse sooner or later. You simply can't do that forever.
GDP per capita and even GDP per capita adjusted for PPP is also misleading because it uses the wrong basket of goods: focusing on things that dont matter.
People forget, wealth is not money. Wealth is the things you buy with money. Wealth is when you can afford the essentials of life: Shelter, food, water, medical insurance, education and transportation (with the costliest ones being Shelter and medical insurance). Consider the number of working hours the average person needs to expend to afford shelter and compare that to 500 years ago. I think you'll find that there hasn't been much improvement. In fact, in CA and bay area, I think you'll see that we're working even longer hours to afford basic shelter than 100s of years ago.
Of course we're wealthier. The shelter you get to enjoy today is leagues better than the shelter you got to "enjoy" hundreds of years ago. The net worth of humanity has gone up. Significantly. We have more advanced technology than ever before. Whatever GDP is measuring probably doesn't consider the fact that we have compounded knowledge and given ourselves things our 100 year old ancestors could not even imagine.
I'm so glad I'm alive at a time further into the future. I may not know how glad I am, because I may not be able to fully comprehend the loss of all of the things that I have today that were simply non-existent back then.
Those that argue we're no better off than we were 200 years ago are pretty unpersuasive these days.
I'm more worried about the next 200 years though. It took several thousand years of civilization to create the industrial revolution, and that was quickly followed by the information revolution. Both dramatically improved quality of life.
But what if this is the end of the exponential? What if Moore's law is dead, and breakthrough technologies like quantum computers or new propulsion systems aren't just "5 to 10 years away", but decades or centuries away?
There is a lot of evidence today suggesting that the industrial and communication revolution was a blip, closer to a step function in human development, preceded by slow linear growth and possibly followed by slow linear growth.
I'm not certain that this will indeed happen, but if it does, our entire society and world is in for a complete shock.