The realization that you achieved as much as one could hope to achieve and you still didn't find contentment.
That's honestly, to me, the saddest thing about the mega-rich. They don't know when, or how, to stop. There's no goal state where they can say "ok I made it, I can turn it off and enjoy life now, or at least stop fighting tooth and nail to hoard even more bigger numbers."
The mid-life crisis insecurity doesn't have the hope and the rush in the same way. It has a lot more dread and angst.