Opening the "Gates" to wealth tax.

If you tax the Gates' wealth, because they spend almost none of it on themselves, the economic burden of the tax must fall on the groups that are benefiting from their vast saving/investing, which is overwhelmingly what will have to decline.

(Let's set aside their donations for now.)

Other rich people lose out from their saving as it lowers investment returns for them.

The people who benefit are those who e.g. gain from the government getting loans so it can spend more now, or are hired by, paid more, or buy the products of firms that have borrowed his money one way or another in order to start or grow more quickly.

Globally we have a glut of savings that is making marginal investment returns pretty low, so maybe it won't matter economically all that much. At least unless you go a long way.

And the government might use that productive capacity more usefully than just generally increasing investment in businesses.

But either way, the Gates family would only end up paying something like 1% of the tax personally.

(Personally I'd probably benefit from billionaires saving less because it would lead to more rapid compounding of any future retirement savings.)