You don’t raid old drives as it creates cascading failures because recovering from a failed drive adds major wear to other drives

Only if you have low redundancy. RAIDZ is better about this isn’t it? And Backblaze goes a lot farther. They just decommission the rack when it hits the limit for failed disks, and the files on the cluster are stored on m of n racks, so adding a rack and “resilvering” doesn’t even require scanning the entire cluster, just m/n of it.

This is less of a concern with RAID 6, and especially in Glacier's use case where reading any piece of data happens seldom, I'd expect it to be fine.