"Most"? Certainly "some" - most is debatable. I'd like to see some numbers. There is certainly no shortage of super-rich who hit that point and... just keep going, because for those types it was never about the money - it was about the power. And while more wealth might give diminishing returns, power really doesn't, yet remains directly proportional to wealth.

(For me, this is the best reason to tax the rich - it's not about revenues per se, rather it is about avoiding extremely dense accumulations of political power.)