Perfectly substitutable currencies have an exchange rate of either zero or infinity. If there's no reason to hold one over the other, you've no reason to ever give up your holdings of one or the other...until there's a slight imbalance, at which point 1 is clearly superior and you've a huge incentive to abandon ship as fast as possible.
In practical terms, government money is backed by the necessity of extinguishing tax obligations and debts within it's jurisdiction, so it's not perfectly substitutable. Bitcoin has no such benefit - there's never a reason to hold it because everything provided by it can be provided by other currencies. Of course, the fact exchanging out requires selling to a greater fool provides some stability, but therin lies the nature of a bubble - once people want out, there's absolutely nothing that's going to convince anyone else they should want in.
Of course if you hold Bitcoin, for any reason, you need to aggressively argue against this problem - and indeed all problems with Bitcoins, at least until you no longer hold Bitcoin.