What if I or someone else came up with some super-secretive procedure for taking a phrase and turned it into a 256-bit private key. Since the method is not published and only exists in their head, although they would have certainly tested it on a computer at some point, as long as they remember the procedure and the passphrase, I don't see how someone woudl be able to crack that.
It's more likely they'll lose access to their coin because they forget small detail of the parameter/protocol specification or bug within implementation. They also need to keep the implementation software since it's unlikely they'll rewrite it when they use different device, which open new vulnerability.