I see no world were this doesn't end poorly. You either see mass emigration of the wealthy at old age or an incentivisation of illicit off the books banking (both domestically & offshore). I can't think of anyone who'd be willing to hand over the majority of their wealth to the government upon death. It would become common practice to attempt to circumvent this.

If everything above 10 million is taken as a death tax you can be sure no estates are going to be worth over 10 million publicly.