It is common for someone to start in the bottom 10%, work their way into the top 10%, leave a fortune to their children, only to have their great-grandchildren fall back into the bottom 10%, at least temporarily.
Assuming the people in the top and bottom are always the same is a very bad assumption.
It is not the money so much that has created the problems we face. It is the desire for political power, and the changing of the ruling class from predominantly Christian to predominantly atheist/agnostic/pagan.
When your ruling class despises you, and believe a great reduction in population is necessary, bad things happen.
This is exactly the point. A fair percentage of people who are in the bottom 20% are in the top 20% twenty years later. There is A LOT of income mobility in a free market economy.
A good read is "Economic Facts and Fallacies" by Thomas Sowell.