Well, it isn't very rational to collect pile of money and then just sit on it until you die. At some point these early adopters are either going to sell, or use their bitcoins.
If nobody is willing to sell, then there is no trade. That makes a currency worthless. Bitcoin is even more vulnerable that gold in that regard, because if Bitcoin stops being used as a currency then there will be no incentive for anyone to run the Bitcoin software -- and then Bitcoin will just die.
Collapse will come when no one is willing to buy (e.g. crypto is broken or there is no way to spend/use it). As people anticipate this coming, they'll start selling to anybody who is still willing to buy. The price will drop until no one is willing to buy, at which point the value of commodity will become zero.
On the other hand, if many people are willing to buy (in expectation of future increased usefulness), then the price will go up until no one is willing to sell. At which point every holder anticipates some future benefit, cannot buy more from anybody, but has plenty of people willing to buy from him for some price. This situation will stay like that until anyone decides to "use" money by selling it (e.g. to buy a dinner because people need to eat sometimes). No collapse here.
Of course, if everyone does this, the currency will collapse, but that is just the beginning of Bitcoin's problems.