[ThrowawayR2]

Excepting a few philanthropists, they would leave whichever nation that passed such a law, bringing their influence and power to a nation that doesn't, and / or funnel all the excess wealth into overturning or subverting it, of course. I mean, wouldn't you?

Even if the ban were universal, money at that scale is power. Alternative social or economic constructs would form to allow the ex-billionaire to continue to wield that power, e.g. some form of trust or corporation that the ultra-wealthy don't own but does their bidding or some form of clan or dynastic structure where family members and trusted retainers each hold part of the wealth but coordinate their actions.

[Bodhisattya]

This. However, it should reduce power being wielded by people with money alone and incentivize wealth being spread more evenly. If some one has to spread their moolah to the their twice or thrice removed cousins, I would still consider an erosion of power and a better distribution of wealth than before.

[giantg2]

Seems similar to many politicians donating to each other's personal charities.