> My point is that until those investors exit, there is no ROI.
Ok well they can just exit in private markets before these shares are "dumped" on public markets. Therefore there is an exit and ROI. QED.
Anyway your overall point, which was a bad one I'm sorry to say, was about investors dumping shares of overvalued companies on public markets.
You are ignoring things like lockup periods, vesting schedules, and other general machinery that specifically exist to prevent day 1 or short-term dumps of shares. It's not in the interest of the company that is IPOing or the bank - how can the investment bank go to investors and market securities and then on Day 1 those securities (because it's a pump and dump remember?) drop by 10% - 20% - 30% or more. That's bad business and investors will leave investment firms that did that.
When one of these "overvalued" companies IPO (and let's be honest, you don't know how to value these companies anyway so your accusation of them being overvalued is faulty from the start), someone has to buy those shares. If everyone starts selling, the value of the company and the value of the shares drop unless there are buyers. This doesn't really serve anyones interests and even better, you as an individual investor don't have to be a buyer! If someone wants to buy because their own model says it's worth it, that's up to them to decide, not you. Fortunes are made betting against the market (and betting in the general direction of the market). If someone wants to forgo buying, that's fine too.
For investors who don't know about the values or models of valuations of securities they can just take industry standard advice and buy index funds or target-date retirement funds. Stop infantilizing people and assuming that because you lack the knowledge that others must too, or that everyone is just out to scheme and "dump" on public markets, especially without any evidence or without considering how the IPO machinery typically works, who buys these shares, or the incentives.