The coins are just bits on different HDD's somewhere. If that magnetic information dies, bitcoins are lost.
For that to happen, it would require that everyone having the chain stop using the Internet or at minimum the Bitcoin P2P software all together. What would lead to that is another story. Cataclysms, ultra depreciation, laws and regulations...
Also, when all the 21 million coins will be minted, any transfer will deduct a fee, but no more coins will be generated to cover that. Will that lead to destruction?