The goal of a borrowing tax would be to prevent someone with a a $200 mil stock portfolio living off the "buy, borrow, die" strategy and not home equity loans on mere middle class millionaires.

Capital gains, for example, on a primary residence already have an exclusion of a certain amount. There's no reason a borrowing tax can't kick in only after one has let's say 10mil in assets or securities.

Heck, you could even exempt primary residences regardless of value, so you should be fine

edit: here's an explanation of the buy, borrow, die strategy for those who are interested https://www.reddit.com/r/BuyBorrowDieExplained/comments/1f26...