[benwaggoner]

You can also store data in AWS Glacier for $4/month per TB if it's really important.

It's pretty challenging to reencode sources to a lower bitrate while preserving all the quality needed to use it as a source in the future.

Using HEVC likely would allow some more savings, and x265 now has the ability to reuse information in a source H.264 bitstream to speed up a reencode. I suspect that this process would help maintain accuracy to the source as well, as encoding would be refining from how it was encoded as well as the actual output pixels. But I've not tested this archival case myself.

It's a fun question, but the effort to verify you have a process that preserves the needed detail is going to be way more than the cost of just storing what you've got, even if you just pay yourself minimum wage.