I think the key here is not to just provide a toggle for using Glacier instead of S3, but to have the historical snapshots migrated to Glacier from S3 and deleted every 90+ days.
I will gladly pay more money for Arq again.
I will gladly pay more money for Arq again.
Arq is incremental backup. The data structure is similar to git's http://www.haystacksoftware.com/arq/s3_data_format.txt
Unless your files change a lot, keeping the latest backup version in S3 and previous versions in Glacier would mean that most of your backup data are still in S3 I think. Right?