Goes to show. If you say something dumb enough, you can get certain people to believe it without question, and get them fired up.
There are no billionaires buying $250k houses. There are no $250k houses with Zillow estimates at $500k. And most of all, Zillow estimates are not fair market value. But sure, lets get fired up.
That said, taxing unrealized gains aint ever gonna happen. But taxing turning unrealized gains into income might (i.e. structured financing of unrealized gains to extract their value without selling) could, and in many cases already does happen (too tight a collar and you get a tax bill, ffor example).