Yes, "collateralization counts as realization" is the bare basement minimum of what we should do to fixed up the tax code, but I'm less offended by the scenario you described -- which involves skin-in-the-game capital allocation decisions, the whole point of capitalism -- than I am by the far more common situation where the assets just sit and grow and are rewarded for their sloth by a complete absence of tax on the one activity billionaires are best at: sitting back and getting paid for being rich.

Property tax on stock is a better place to aim.