I don't think it will ever fail. It has a big userbase, and people are all the time creating new uses and applications for Bitcoin.
You shouldn't think like this with the
it's too big to fail line. That's what they said about the banks. If bitcoin was too big to fail the price would likely not be so low as it is now. If bitcoin kept falling it would be too expensive to support so it'd fail if all the miners left or moved on to something better so bitcoin can always fail though I sincerely hope it doesn't.