Why do you think demand cannot be sustained ? Bitcoin is a payment system. If you want to use it you need to purchase some bitcoin. The amount of bitcoin is largely irrelevant. You purchase for a certain value. If the value keeps going up you just get less bitcoin for the same value. But you can still use it for transactions for that value. Bitcoins are basically infinitely divisable. The current minimum amount of 1 satoshi can easily be subdivided.
When you think of bitcoin only as an investment vehicle it's true that is has no value. The value is generated by trade in the form of transactions.