There's less risk of damage from running off a single disk in a faulted mirror than to rebuild the mirror. The stress the drive is under to rebuild can and will kill drives at the end of their life.
With btrfs, you can easily add a new drive so that new writes can be accepted and mirrored immediately, then start rebuilding the old data with lost redundancy (possibly after running another incremental backup, which will be less stressful to the drive than the full rebuild). But the "add a new drive" step happens outside the kernel, in userspace and possibly in meatspace, so it can't be a default action for the kernel to take.
Also, if you're happy running off a single disk in a faulted mirror, then I'd question why you've got the mirror setup at all.