Back to the topic: I think that at some point all the willing investors will fulfil their desire to buy bitcoins (especially because the price of bitcoins is going up too much) and at some point the trend will invert itself and because of it with the help of some panic sell the price of bitcoins might go under the reasonable value
This brings new questions to mind: When is this desire first fulfilled (market capitalization)? Why would investors (not just traders speculating on BTC/fiat-exchange rate) heavily sell if Bitcoin's still the "best" currency? Wouldn't traders reinvest if exchange rate falls below "the reasonable value"?
Even if BTC is the best of the best currencies of the universe there must be some physical limit for it to go up:
1) the supply of the investors has a limit (at least the rate of their increase)
2) their means are limited too
3) and even if BTC is a good currency you need fiat money to eat.