I think people talking about how much they would buy if it dropped to $300 take for granted that it will automatically rise again in the future. Nothing works that way. There are no guarantees. If the price dropped so severely, it is probably because the future outlook has been greatly diminished.
I am not believing in exact repeating of history. Because the user base and investors and their investment volume are few significant factors must be differing from last time. Even some part of people get panic on a slide, I believe rest of people will be backing bitcoins by NOT selling all the bitcoins they are holding.
Sell everything is not a recommended action for any kind of market situation. Because it will throw you out of race within seconds.