Bitcoin has two qualities that are unique:
1) Extremely high degree of privacy (you can put 50.000.000 USD worth of USD in a USB stick and pass through 7 airports, or print them in an A4 page encrypted with GPG, and no one will know).
2) Transaction speed: You can send money from Iceland to China (even huge amounts) very quickly (less than 1 hour), with no third party being involved.
There will be always a market to request this kind of qualities. However if any of those two qualities goes missing for whatever reason it is doomed. Another way to kill BTC would be to create another crypto-currency that have additional features and would kill BTC on the spot.