If top-tier companies don't go public, isn't buying index funds a waste?

Buying a broad-based index assumes that among hundreds or thousands of listed companies, I don’t know which ones will ultimately be successful, but the successful companies will go public, grow into winners, gradually become large companies, and enter the index. By buying index funds, I can share in their profits and growth.
However, what if the successful companies never go public and instead beat up all the listed companies, causing profits to decline?
Then, investing in the index would be pointless.

For example, if Huawei (hw) never goes public, the impact seems relatively small.
But for ByteDance, the impact on Chinese concept internet companies is significant.

People say that going public provides more financing channels and stronger capabilities. So why are some companies that don’t go public still stronger than listed companies worth trillions?