Some potential scenarios are: * A security or encryption issue discovered in the protocol that leads to compromise of most wallets. The bug could be fixed, but

Some potential scenarios are: * A security or encryption issue discovered in the protocol that leads to compromise of most wallets. The bug could be fixed, but existing balances and trust may be wiped out. * Government regulation that makes it illegal to exchange bitcoin for cash or goods at legal businesses. It may remain as a niche underground thing, but even drug dealers need a way to eventually buy something else with their bitcoin. * Insecure exchanges that make it impossible to exchange bitcoin for cash. Exchanges seem to get hacked and disappear even without government "help". The result is the same: people need a way to use bitcoin for it to have any value. * Another digital currency becomes more popular. * Someone with enough computing power takes over the network. This could be a government agency aiming to shut it down, or perhaps a back-door in ASIC miners.