That's basically a recipe for rich people never payping tax again. They pretty much do this already. It's called "buy, borrow, die" [1]. Basically, you borrow money against assets and spend that instead of realizing a profit or gain and being taxed on it. You basically never pay off those loans.

Also, this claim is factually untrue. We tax property based on unrealized gains all the time. There's absolutely no reason why we couldn't do this with other assets.

[1]: https://smartasset.com/investing/buy-borrow-die-how-the-rich...