The lives of billionaires, despite their toys, are far more stressful and busy than any of ours is. Their day-to-day lives are probably not very fun. It's not envy.
There comes a point where you have so much money, your life isn't just unlikely to be improved by more, it's impossible to be improved by more.
What is the harm of a 100% wealth tax on monies made/earned/created over some huge number, like $1B?
Once you have that much money, the only way it won't provide for your family for generations is sinking it into a bad business or investing it poorly.
One can just do any of the 99.999% other jobs/activities/whatever with their lives and not need another penny for life.
On the end of a bell curve, things become skewed - once you're like 7 SDs from the mean, none of it matters anymore.