Consider that less then fifty(!) people are as wealthy as the poorest 50% of the world's population combined. Back in the early-20th century the whole "some rich or everyone poor" argument held merit, but with the extreme wealth accumulation of the top 1% today that's no longer true. If that wealth alone was to be distributed evenly around the world it would major increases in living standards for billions of people without a perceptible drop in living standards for everyone but those 50 people.
If we go wider, there's two thousand dollar billionaires with a combined wealth of 8 trillion (that's 8 million millions) dollars. Now, if we distributed that evenly that's only a thousand bucks to each person in the world. But if that was to be redistributed to third world countries, we'd be talking tens of thousands of dollars each to people who earn maybe a thousand or two a year. Now, two thousand people is not even close to the 1%, so there's plenty of money to be tapped from the 400,000 richest people in the US or the millions of people worldwide who comprise that 1%. My point is not that this should be done, but that the richest people have accumulated staggering amounts of wealth, more then enough that it could be spread around without making "everyone poor".