I think the main argument is that the French billionaires are much more mobile - they were able to simply relocate a few hours away to a nearby French-speaking

I think the main argument is that the French billionaires are much more mobile - they were able to simply relocate a few hours away to a nearby French-speaking country. Any sensible wealth tax proposal in the U.S. would probably have to pair it with a similar tax on the transfer of assets abroad.