The billionaire stock owner in your example's economic power has increased by 10x regardless though. They can borrow against that money and those funds aren't taxed - so they can effectively spend it without taxation.

They can cycle debt to avoid ever having to sell a single share for their entire life, at rates more favorable than any middle class person has access to.

We can fix our tax code with legislation to address this loophole, our politicians just don't have the will to do this currently (largely because many of them amass fortunes through stocks and insider tips, IMO).

The thing is, when most of your money is in illiquid assets (stock, in this case, or houses, etc.) you can reliably get an extremely low interest rate loan against those assets. What does that mean? Let's say you have 10B equity in a company, and you want to buy a used 747 Jumbo Jet--a steal, at $500,000. You only have about $100k liquid cash on you at the moment. No problem! Go to a banker, take an extremely inexpensive loan out against your 10B (think less than 1% interest), and buy your jet. Because it's not income, it's not taxed. But yes, these people do actually have access to that much purchasing power, it's not just abstract numbers.