What are you talking about? HFS is creaky and ancient and doesn't have checksums, ZFS and btrfs do.
HFS+ is creaky, ancient, but battle-hardened.
ZFS is solid and battle-hardened, but not that suitable for many of Apple's customers, who want the ability to unmount disks they added, and who will not understand that they need huge amounts of RAM (they their Macoboks don't have) to keep their disks speedy,
btrfs is promising but not battle-hardened yet, and it is very unlikely Apple will use it as long as it has a GPL license (I think it is more likely that Apple will add checksums to HFS+ in some somewhat clumsy, but working, way than that it starts supporting btrfs)
If you think HFS and HFS+ are battle-hardened I don't know what to say to you. "Shell-shocked" would be a better description.
You're a very lucky man, then. A large part of my time, at one point in my life, was spent fixing up corrupted HFS filesystems.
I would generally agree that it's rare for e.g. HFS to corrupt its own metadata due to bugs but bit-rot happens. If you think you haven't encountered it, you probably haven't actually checked your files closely enough – this is particularly easy to miss with things like video or image formats which were designed to tolerate a few bit-flips.
The reason why so many Mac users were interested in ZFS was that a modern filesystem incorporates strong integrity checks so you are guaranteed to either read back the bytes you originally wrote or get an error. That's a much better way to work than requiring you to use a tool which does strong hashes periodically to see if anything has broken.
A lot of consumers are quite obvlious to bit rot. As you point out - most of this occurs in large media files which they are unlikely to notice. Even with things like corrupted documents they will likely blame it on an application glitch.
With ZFS they will see scary diagnostic messages. Consumers will want their SSD or RAM replaced. (ZFS does a great job of catching RAM errors on systems without ECC)
This would no doubt drive higher warranty costs for Apple and cause consumer dissatisfaction.
Ignore is bliss...
I'm skeptical that a proactive warning would be more dissatisfying than just finding it out years later, when other copies are harder to locate. In particular, I'd be surprised if a hypothetical feature like this wouldn't be integrated with their other services: “A local failure was detected and replaced with a pristine copy from iCloud”
The other really interesting question would be whether they'd ever consider something like the ZFS copies feature to directly trade maximum disk capacity for redundancy. That'd be much easier to implement for Apple since they control so much of the platform and the largest files which most people have tend to be things which are provably known to be local copies of something which can be re-downloaded, so the total cost of doubling storage for unique local files might be worth the peace of mind for many users.
I suspect the decision not to use ZFS was (assuming it wasn't simply spite at Sun pre-empting Jobs' announcement) that it would cause dissatisfaction, but from a different direction: ZFS was designed for Sun's customers: companies employing dedicated sysadmins. ZFS was incredibly unfriendly back when Apple would have been evaluating it. User experiences like "you filled up your filesystem? Oh well, you'll have to reformat and restore from backup." may be acceptable in an enterprisey setting, but aren't going to cut it with the iMac crowd.
In the same logic people had issues with ZFS too.
"With issues" means nothing. The number and severity of them only has meaning, and even that only comparatevely.
There are multiple businesses which sell software which exhaustively attempts to recover data from corrupt drives & cards.
It's a standard service listed on the local phone / computer shops.
I see messages on my neighborhood's mailing list every so often from someone looking for help recovering as much as possible from a flash card.
It's simply absurd to claim that the status quo works for the general public. It's not terrible, probably not that different from various mishaps which befell printed photo albums, etc. but there's still plenty of room for the industry to give people safer tools.
Also, why wouldn't you be able to unmount a ZFS disk?
zfs unmount ...
Generally though, you wouldn't want to manually unmount file systems unless you're planning to migrate them to a new host. In which case you would export them instead:
zpool export ...