> because once you reach a certain (very high) level of wealth, there’s the “buy, borrow, die” strategy that avoids realizing most of your capital gains

If that is what is being targeted, then why not actually target that. Apply some percent taxation on the current value of all assets transferred because of death. And, if they want, only apply it to estates over some X threshold in size.

Performing the taxation at time of probate makes the valuation easy (unlike a 'wealth tax') because the valuation could be one of "value at time of death" or "value at time of transfer". And, if the ultra wealthy are using this angle to avoid taxes, then this taxes some of that transferred value.

Of course, just like with subscriptions, to the politicians a yearly wealth tax is far more valuable than a one time tax on the total value of the estate.