Are these good examples, though? Dropbox is down ~35% since IPOing, and Slack is down almost 50%. This certainly suggests the market thinks they were both overvalued when they IPO'd.
This is arguably a "things might go wrong in the future" argument, which has no real substance as it can be made about literally anything. To really say something, you'd need to provide some more substance about why you think the current investors in Dropbox and Slack are less informed about economic conditions than investors in the rest of the market.
> The Fed will run out of monetary tricks, eventually.
No, it won't.
It might run into a monetary-policy resistant situation (e.g., stagflation), but the Fed has infinite range of monetary policy tricks available (literally, there's no floor to rates now that the Fed has taken notice of the use of negative rates elsewhere.)
Slack went public at $26, it is at $23.50 today. It is less than 10% less than the IPO price.