It is technology that is the primary culprit.
In a few years, about 50 million workers will be able to make all the goods the world requires, including food.
Then what will everyone else do?
Jobshift: How To Prosper In A Workplace Without Jobs - by William Bridges
This has happened so many time in history that we actually know a definitive answer: the rest will also be workers but producing the kinds of things that do not even exist today. What happened to the huge steno pools and then pools of typists that every corporation had just a few decades ago? Some became computer programmers, some others became administrative assistants.
Similar predictions are periodically made about the shortages of food on Earth and the inability of mankind to sustain itself, but those predictions are based on the inability to envision that new methods and sources of production will become available. That is what happened in history time and time again.
Namely "telecom", or simply the internet. White collar jobs can be done from anywhere, so Chinese and Indians don't even have to come here to take our jobs.
When I look at my sixties era economics textbook ( Samuelson ) it's evident that this eventuality falls outside the models considered there. Commerce between nations was assumed to consist in the trade of material goods.
In a few years, about 50 million workers will be able to make all the goods the world requires, including food.
Then what will everyone else do?
Invest! That seems to be the idea, but markets are volatile and capricious, especially in the circumstances you outline, where everybody is a capitalist and very few actually produce. Of course, we're seeing this now. Things really are changing fast and I don't think anybody knows how it's going to shake out.
I just hope we don't drift into monolithic world communism, which seems to be the liberal ideal these days. To me it's supremely ironic that liberals are pushing so hard to create the totalitarian nightmare that they always thought they were fighting.