Well, on one level, yes, but in practice there are many organizations and governments that would step in if a national currency were headed towards a value of z

Well, on one level, yes, but in practice there are many organizations and governments that would step in if a national currency were headed towards a value of zero. Bitcoin doesn't have the same level of institutional support (yet?), and so if it's value starts crashing hard, it could turn into an unrecoverable death spiral.