For me, it's a short list.
- An effective attack on the crypto - partial preimage attacks on 2SHA256, preimage attacks on RIPEMD160, forgery attacks on secp256k1. In this situation, the ledger becomes unusable and it becomes a game of he-said she-said to get a "clean" snapshot for forking to sounder algorithms.
- A 75%+ hashpower vandal able to maintain that position for months. With a clean supermajority a bad actor can keep parallel blockchain branches within 6 confirms of each other (thereby keeping the ledger from converging and making it unusable) as long as they maintain that supermajority.
- System failure of the Internet. Bitcoin is useless without the gossip network; we've discussed how to theoretically fail over to HAMs but I doubt that'll ever get built.
I think Shamir User A dumping their 2.8MBTC would push us pretty low, but it'd be temporary. I think the development of a significantly better Bitcoin-like system would cause most people to jump ship, but there'd be folks left who'd keep swapping them as collectibles. I think successful, high-profile reorg attacks ("51% attacks") would shake confidence in the system to its core, but that's a wound that the network can heal by changing its behavior. Those are all scenarios that could conceivably push us below the $2 wall that I've (colloquially) referred to as impenetrable, but
zero? Zero is a
long way down; even those debased Zimbabwe Dollars go for a couple bucks a
bill just for the novelty value. Bitcoins becoming
worthless would imply, at least to me, that something had changed such that ownership itself couldn't be established anymore.
