There nothing mathematical about guarantees for bitcoins continuing existence — it really boils down to the social contract between miners that agree to use the same set of rules. See for example the result of the Dao hack in Ethereum world — if there really was any mathematical guarantee to Eth smart contracts, you’d imagine you couldn’t simply revert them by updating the software. The parallel to Egan’s dust theory is interesting, but only valid as long as someone is actually mining blocks using old clients nobody cares about in the long forgotten forks of the chain, which doesn’t really happen.