If in that 300,000 years the user decided to change the encryption key you would have to start over again. Obviously the 300,000 years could be made even longer by adding more digits to the combination, but the advantage is questionable. Finding out the combination directly. Users have been known to share their passwords. Statistical attack on the encrypted data.
Statistical attack is the real proof of an encryption method's security.
A statistical method does not attempt to try every locking combination but instead attempts to determine the pattern of bit changes needed to reduce the randomness of the file. Poor encryption methods such as XOR masks can be decrypted rapidly with these methods. More sophisticated methods require more time. Although mathematicians can make intelligent guesses about the relative security of different encryption methods, the real proof is in the test of time. If many mathematicians working over many years cannot do better than trying every combination then the encryption algorithm is considered secure.