It isn't user error if it was working perfectly fine until the provider made a silent change.
Unless the user error you are referring to is not managing their own backups, like I do. Though this isn't free from trouble, I once had silent failures backing up a small section of my stuff for a while because of an ownership/perms snafu and my script not sending the reports to stderr to anywhere I'd generally see them. Luckily an automated test (every now & then it scans for differences in the whole backup and current data) because it could see the source and noticed a copy wasn't in the latest snapshot on the far-away copy. Reliable backups is a harder problem then most imagine.
Also consider e.g. ~/.cache/thumbnails. It's easy to understand as a cache, but if the thumbnails were of photos on an SD card that gets lost or immediately dies, is it still a cache? It might be the only copy of some once-in-a-lifetime event or holiday where the card didn't make it back with you. Something like this actually happened to me, but in that case, the "cache" was a tarball of an old photo gallery generated from the originals that ought to have been deleted.
It's just really hard to know upfront whether something is actually important or not. Same for the Downloads folder. Vendor goes bankrupt, removes old software versions, etc. The only safe thing you can really do is hold your nose and save the whole lot.