If we extend this thought experiment to a physical key, then that argument falls apart. A physical key is 'real or physical evidence', while a password (and apparently combinations?) ends up being considered 'testimonial', despite both serving the same function. While I may not be required to provide testimony against myself, I can be compelled to provide real of physical evidence. If, for instance, I have committed tax fraud, and my accountant has already told the IRS as much, but I have the only copies of the tax documents encrypted, I can be compelled to decrypt them, because the 'testimonial' value of the password is negligible since the gov't already knows that I have the fraudulent documents. But if they can't already demonstrate that they know what--roughly--the real evidence that's encrypted is, then no password for cops.
This seems inconsistent to me, since a password and a physical key serve the same function.