If Bitcoin loses 42 billion market cap, it will probably not flow down to altcoins, it will go back into the Dollar and Euro.
It's hard to believe this. The demand is so huge and it will be even bigger when people start to really move to better currencies. Currently Bitcoin sucks very much because it's so slow and expensive to use. When the network effect shifts to better working alternatives, people start to actually use them again as currencies and we will see even bigger boom.