Is the net worth of someone like that really a valid concept?--since if they announced that they were to sell all of their stock, the price would plummet (since

Is the net worth of someone like that really a valid concept?--since if they announced that they were to sell all of their stock, the price would plummet (since it would imply to investors they no longer trust the stock as an investment). It's worse than illiquid; the mere act of trying to sell such a horde would destroy its value.