[ebbv]

Were you not around for the first dot com bubble?

All it takes is the right high profile failure(s) and investors get spooked. They stop investing, which causes more collapses of companies that can find investment. They pull their money out, the selling undermines market confidence and things go kablooie. That's just one example of how it could happen based on how it did happen before, not a prediction of what will happen.

The valuations are based on confidence, if the confidence is reduced (not even entirely gone) it can start a downturn. That can go fast or slow.

[junkscience2017]

but in the first .com bubble, companies lived and died only on their stock activity. these days, the winners have enough cash piled up that a recession would actually be welcome; it would allow them to buy companies and employees on the cheap, and many of their flimsy competitors would fold.

if you think Apple, Google and Facebook dominate now, wait until the next recession. can your company laugh off losing half it's market cap temporarily? they can

[ebbv]

Apple, Google and Facebook are not the issue dude. All the companies that have yet to really be profitable are. Come on, that's obvious. Of course those companies which have cash on hand would be fine.