Now you are mixing up income / capital gains tax with property tax. Property tax is a tax levied by some local governments. It is not based on income, but based purely on the whole value of the real estate you own (not on the gain) - and it is a much lower rate than income tax (if it was too high it would force people to sell their homes to pay the tax).Please see my edit as you do in fact pay taxes on increased assesment of property. At this point no matter what you say will change my mind as to the need to tax billionaires that use stock as a tax haven.
I'm not trying to change your mind, I'm just trying to point out that is not a straightforward as you make out. You have not explained, for example, what should happen if Tesla's stock were to drop by 90% due to a competitor successfully overtaking team as market leader, or someone inventing a better battery. This scenario is not far fetched, it has happened before. What should the tax man do then? Give Elon a tax rebate?
The real issue here is that entrpreneur billionaires are really not as wealthy as the press makes out. They have an asset that is superficially given a $ value by a market - but it is not really worth anything other than control over the company's future. They do sometimes sell stock (those loans need to be serviced!) and when they do sell stock they do pay taxes like everyone else. Ditto when they exercise options. But they can never convert all their shares into cash. And the companies they run provide jobs which generate all kinds of indirect taxes. To tax them based on the value of their company would force them to sell stock to pay the tax - thereby eroding their control over their company over time. How is that fair, or a good idea?
Bottom line is: your idea that stock is used as a tax haven is completely misplaced.