The case law on compelled decryption is less clear-cut than you're describing. (Possible exceptions include: biometric, as opposed to password/passcode, encryption; and the government being able to prove that the defendant knows the password.) Moreover, even when the government can't obtain the decryption key from the defendant, cracking the encryption is not the only possibility. They could also seize the device while it is running, with the decryption key and/or decrypted data already in memory. That was done in the arrest of Ross Ulbricht, for example.

Also, this is entirely different from your original claim, because it has nothing to do with where the encrypted data is stored. If you store encrypted data in the cloud, then the fifth amendment provides you with the same degree of protection from disclosing the decryption key as it would for locally-stored encrypted data.