I simply disagree that an economic incentive is necessary or useful for people with extreme wealth. Already rich people who are primarily motivated by the accumulation of wealth are unlikely to have some unique skill that society needs in our active labour force. If our tax structure made them more likely to retire early, our workforce would not be starved of useful skill.

Genuinely smart people with genuinely valuable skill (like Musk or Jobs) would work equally hard whether their offspring were set to inherit 10 million or 10 billion dollars.