Or will they feed off of Bitcoin's carcass and take off?
If bitcoin goes down everything goes down, we have been able to see that the last few days and also many times before. For many still alt coins cannot succeed without bitcoin, hopefully in time this mindset will change. Also though let's not forget that many times what causes one to fall also causes others to fall, such as the current reason for the fall.
Its not in line, in some cases if bitcoin drops some altcoins pump. Yes we see it the changes happen not all succeed in this market. So better ride just to earn profit and ofcourse out if you are earning already.