The richest people I know or meet always tell me the same thing: prime cities in the developed world simply can't go down!

New York, London, Paris, San Francisco, Vancouver, Singapore, Sydney – they're all invincible! Or so these guys would have you believe.

If there's any two things history doesn't support, it's that the biggest cities always reign supreme… and that the ultra-rich always remain rich.

There's a reason all good mothers tell their children to become doctors or lawyers. People in those professions will likely end up in the 1% to 10% bracket where they make their money more systematically – like from high salaries. They might not make it into the top 0.1% to 1%, where incomes and net worth are totally unstable, but they'll make a hell of a living, and have stability to boot. The guys at the top? They may get rich off some radical new business or IPO or from Wall Street speculation. But they might burn out just as quick.

That's just how they work. Radical innovators, speculators, and even conquerors “live and die by the sword.” They're too invested – both emotionally and financially.

I get it. But as one of those people who started a radically different business and took huge risks, you'd be shocked by how much my income and wealth has varied over the last 20 years. You're not on top forever!

So for these people who think they're going to be rich forever and that they can park millions in cash in prime real estate and get away scot-free, they've got another thing coming.

There were two points when the top 1% controlled near 50% of the net worth in the U.S. – 1928 to 1929, and 2007 to 2015. The very top, the 0.1%, control as much as 25% at these peaks.

But those peaks don't last, as the bubbles that create such extreme wealth don't last. Their wealth tends to get cut in half in the years and decades to follow.