Nobody cares about data junk. Especially your personal data junk if it is all encrypted. I don't think that a lot of persons will look at your data there. If this sort of good encryption you consider for public cloud backups breaks we have a lot more problems than exposed backups.
You want math? How many combinations are afforded by your "long, carefully chosen password" in a symmetric system? How many core seconds per hour does a typical botnet scriptmonger control? Cryptanalysis of GPG doesn't even matter if Eve has enough time to brute force your symmetric key.
