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."
Point is that competition is fueled by insecurity. If you are happy, truly content, with yourself you will find competing extraordinarily tiresome and unnecessary. Not saying it’s good for society by the way. I think our civilization needs the pathologically insecure to be disruptive and create room for innovation. Related traits are narcissism and psychopathy. Painful, but useful, in small doses.
Musk, Altman, Bezos they are basically caricatures.
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.