If you truly believe that the stock isn't worth 50B, you are free to short the stock. In fact, lots of people out there are shorting the stock. However there is no smoke and mirrors here: they release quarterly financials, they've been public for a boom time and a down time. The aggregate of the market disagrees with your view.

The OP also specifically says the point of the charade is to "make huge amounts of money for the founder and the initial VCs". Uber/Doordash didn't just make huge amounts of money for the founders and initial VCs, it made huge amounts of money for early employees, later employees, current employees, the initial VCs, the later VCs. People have had years to exit their positions and unlimited liquidity to do so... some other rational actor was happy to buy the shares they were selling. If you say these companies are charades than so is every publicly traded company.