Who will buy the phones when no one is paid to produce them? The cars? The food and the clothes? Wealth keeps concentrating upward, great leaps in efficiency and throughput, but I can't see how it's a sustainable model for continued consumption growth year after year.
Other wealthy people. I knew someone in the yacht-building business who would say "If you want a business that will last, sell to rich people--they're the ones who have money." We are very quickly moving towards a world where the economic activity (earning + spending + producing) of the median person is insignificant next to the activity of the very rich. There are individuals who have more wealth than the GDP of entire countries.
We're bifurcating into a society like the movie Elysium: A relatively small number of wealthy people who matter to the economy, and a huge number scraping by day to day whose economic activity amounts to a rounding error in the grand scheme of things.