and this is completely wrong.
The money public investors pay to exiting shareholders do play a major role in the economy - it enables the early, IPO/angel investors to exit, and allows them to convert capital locked in the established startups to new startups, without waiting to "cash-out" using the company's profits (which may be years away).
In other words, the public markets makes capital movement much more efficient. It lets high risk takers take on bigger risks (for the corresponding potential return), without taking up the required time.
It's a misconception that many people have, that investing in the public markets is less useful to the economy than direct investment. Both play a critical role, and without one or the other, the capital markets will be _way_ less efficient.