I'm going off of survey and financial institution data, which shows that 25-40% of portfolios for HNWIs are in cash equivalents. Sure, there are exceptions. But I've got some familiarity with this topic and I can assure you that it's not a "mistake" than I'm making. I think you're confusing anecdote with data - the bright examples are "sticky" and color the bias.I think you may be making the same mistake a lot of people make when discussing wealthy people. Entrepreneur billionaires don't tend to hold a lot of cash on hand and their investments are not very diversified. A couple of articles
An entrepreneur who interviewed 21 billionaires says there's a common misconception about how the world's richest people spend their money
Why Elon Musk Is Cash Poor (For A Billionaire)
People who invest for a living are constantly moving money around and most take advantage of the games that have been set up like buying stocks on margin, shorting stocks, etc. But entrepreneurs made their wealth in a very different way and most have very little in the way of liquid assets.
It would actually be foolish for someone like Elon Musk or Tim Cook to cash out a substantial amount of their stock in their companies to invest elsewhere. They remain in control because they own enough stock to prevent a stockholder's coup and their return on their company stock is getting them more wealth building than almost any other investment they could make.
If you have a vast amount of stock you can borrow against it, but you need to pay that back eventually. That could work as an occasional thing, but it wouldn't work as a yearly ploy to pay your wealth taxes. They will eventually have to sell stock to pay the bank, diluting their holdings in the company.
When someone owns $50 billion in stock they can also live quite well selling a tiny amount of it every year. Few entrepreneur billionaires are tapping anywhere near 1% of their wealth every year. And the ones who are usually are doing charitable work with it.
If there was going to be a wealth tax I would hope there would be an exemption for self made entrepreneurial wealth. Hedge fund managers would whine about paying the tax, but it wouldn't really hurt the country if they had to pay such a tax.
The Wealth X Billionaire Census agrees. I'm absolutely open to data that doesn't align with these, but random examples aren't really demonstrative except to be indirectly misleading.
I think the mistake that you're making (since you've introduced that concept here) is that you're assuming that your "billionaire entrepreneur class" is representative of the group.