Any agent capable of ever reading AES encrypted files with 256 bit keys is also an agent capable of opening your safe deposit box, requesting your tapes from a data vault provider, or coming over to your house with a rubber hose.

Short of a vulnerability in AES (in which case we have more problems than a few copies of the Anarchists Handbook in our backup files), cracking proper encryption is simply not feasible.

What about in 10 years, 20, 50?

With arq there are multiple layers that would have to be broken to get to the data. Still a risk, obviously - and one worth considering. I bet in the scheme of things though there are much easier ways to access this data available now.