The assumption is that only a small fraction want to sell. That mentality is driven by other factors. The reverse could be true as well, if 90% of the first 100 want to liquidate their holdings, then the price drops.
This is all just supply and demand curves. That said, I will just hodl my coins.
Absolutely, if something happens, there may be a rush to sell. Imagine if they discover that bitcoin trading stops people from sleeping, or causes them to check their iPhone while working or driving. What if making money causes teenagers to buy a Lambo and crash it into a lamppost. All that could give bitcoin a bad rap.