Wealth gap will make the poor gradually loose enthusiasm for life and for work, they will not want to try anymore, meanwhile, the rich will keep getting richer. And in the end, there will be people in the world who are super welathy, and the rest are decadence poor people, but at this point, the society stops working because there will be nobody working, and the rich can't even buy things or get more money as they have no employees.
Example: South Korea and Philippines, at around 1960, they have a lot of similar economic indicators. But South Korea's economy has developed vigrously in half a century, while the Philippines' economy has stagnated. The main reason is that South Korea's income and land distribution are more equal than the Philippines.