Billionaires have been too and the tax code is one the slowest things to adapt.

In tax code sense I am not even sure the effects of buy, borrow, and die have even been settled.

EDIT: can't respond but "pay no taxes but at least your kids do" isn't exactly a fix

That is a hole we could fix tomorrow by eliminating the step up on death. No need to introduce a new global financial surveillance regime for tracking asset ownership for wealth tax purposes just to fix that.