You don't have to believe. If you have a 401k you will be an investor 15 days after launch. The IPO will go great, because the company will float a fairly small issuance. The big shareholders will not immediately sell. They will hold on and maybe even buy to support the price. Then, after 15 days, it will enter the indexes and everyone's 401k will start auto-buying this stock. You might say this is an obvious flaw in…
This is not a given.
Many people have many different kinds of investments inside a 401k. Your 401k can own a rental property. Or gold. Or, in a more mundane scenario, the Russell 2000.
If it weren't for the glacial pace of plan administrators and plan holding companies there would be an opportunity for a fund provider to offer "S&P500exSpaceX". It's just another index, after all ...