:)If people lose faith in the security of the network for one reason or another then bitcoin could die. no matter how many times i look at the numbers i always have this idea in my head that an address could finally be cracked with a super computer ie one of satoshis old addresses, this would obviously be catastrophic . Maybe after years of not achieving mass adoption price will slowly taper off and die, to me this is the most likely case.
Bitcoin uses 256 bit encryption to secure the private keys that are required to move BTC from address to address. It is highly improbable, that any computer other than a quantum computer, would be able to come up with a valid private key to be able to move funds in an unauthorized fashion, in a reasonable amount of time.
Having said that, within the last couple of years, there have been advances made in quantum computing. Recently, at a University, a working model with 5 qubits has been achieved. This new working model is not powerful enough to be able to conduct the calculations necessary to crack 256 bit encryption in our lifetime, however, it is very significant, in that it has allowed scientists to go to the next step in the field of quantum computing.
We don't really know what type of working model the NSA has. A possible sign that a fully working model may be near, is that the NSA recently updated the encryption it uses to something much more secure. Some people in the field of quantum computing believe that a true quantum computer will be developed within the next 5 to 20 years. By then, bitcoin should have completely switched to a quantum cryptography scheme if it's going to survive.
It is possible that all the bitcoin addresses that are currently locked in the blockchain do to the private keys being lost may be recovered by someone with a quantum computer one day.
With proper planning Bitcoin can survive the advent of quantum computing.