Remember if bitcoin becomes too expensive to use (eg. a satoshi is worth more than a cent) there can always be alternatives like litecoin for the cheaper transactions and bitcoin to store the wealth.
The problem is, if Litecoin isn't so rock solid in value like Bitcoin, people will still want Bitcoin to store their assets.
N. of people trusting in the cryptocurrency is EVERYTHING that gives value to that cryptocurrency.
At least, at the moment, and for quite much time still.