The entire point of my system is that I don't need to care which copy is good. If it scrubs and detects any error, I'll mount disks independently as two filesys

The entire point of my system is that I don't need to care which copy is good. If it scrubs and detects any error, I'll mount disks independently as two filesystems, and diff the contents. I'll then restore corrupted data from backup. Yes, a checksumming filesystem would be better, but at the time of creation (and even now?) none of the filesystem choices were mature, or proven to be sufficiently reliable. Ext4 and software RAID1 are both mature and proven reliable.