The minimum net worth of the top 1% of households is roughly $13.7 million[1]. So at $50 million they can say "we're only taxing the top of the top 1%" as a way to sell it.

"The top 1%" is a popular target for these schemes because 99% of people might be convinced to support it, since it won't affect them (at least not directly).

[1] https://www.investopedia.com/financial-edge/1212/average-net...