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.
You see the problem with this when artificial agents get into bidding wars - because they react so quickly, they end up creating massive price volatility because when it's worth buying its worth buying and when it should be sold they can't dump it quick enough.
In practice every thing has some type of limits - i.e. even computers operate over limited time intervals, and any individual actor has limited resources so eventually something exhausts itself (i.e. everyone might have dumped Currency 1 for Currency 2, but even if no one holds any any more, they'd still be trying to sell it off if they did - ergo, we never reached a finite "price" for it.
The kind of delusion thinking seen here about Bitcoins is akin to the delusional thinking I heard in 1999 regarding tech stock prices. Does anyone remember when Cisco's market cap was over half a trillion dollars? But Cisco actually did (and does) something that creates and stores real value.
There's all this talk of how Bitcoin is easier than this or that currency. Well it's (possible but) not easy to short Bitcoin. I certainly can short the dollar in a number of ways. I'm sure if Bitcoin was shortable it's price would not be where it is. It certainly means the inevitable fall will be harder though.
It's hard to say when Bitcoin's worth will crash, there's a lot of money sitting out there, waiting to speculate on something. But inevitably it will crash.