Dumping the 3-5 million bitcoins that were generated out of nowhere among the "early adopters" and that have never been traded before will cause massive inflation. Perhaps people aren't expecting any earnings but at least they'll want their money to keep it's value.
I think it's unlikely that this would happen all at the same time. Yes, there are several members that have loads of BTC stored in their wallets, but they're all probably very smart people (given that they took the risk to join this project early on, seeing its potential), and wouldn't just drop all 100k+ BTC at once. They might put a cap on the market price for a while with a large ask, but I doubt they would be stupid enough to just sell everything they had all at once no matter how much/little it was worth.
Also, many of these founding members are some of the biggest believers in bitcoin lasting and USD failing. Why would they want to change for a failing or inflationary dollar?
Finally, it's even more unlikely that several of these large-holding members would do it at the same time. Likely, one would wait until the previous seller was finished, so as to not devalue the currency too much.
If such a thing did happen, I'd be one of the first to start dumping money into the exchange buying up cheap bitcoins.
And I'm sure there are many others like me who would help quickly stabilize the price - not out of a feeling of communal ambition, but out of entrepreneurship, and believe that bitcoins will rise back up to their previous value after a quick devaluation resulting from a "dump" of a rich guy.