And I'm not sure you need snapshots and many of those industrial features on a watch anyway. Sometimes simplicity is actually a plus. So who said they need "one shoe fits all" to begin with? It's never a good approach.
You don't necessarily need snapshots on a watch, but I can see it coming in useful on a phone when you're editing pictures and video.
Snapshots on a watch could be useful for system recovery. Say something went wrong during an update you could potentially use the snapshot to revert to a previous version.
On the other hand, computers are gradually moving towards miniaturization, so I suppose all this really is quite transitory, and soon enough all such considerations wills simply be irrelevant.
The main point is that upgrading "in-place" does not require any of the metadata to be in the same place between old and new filesystem. With a mildly flexible destination filesystem, the old filesystem can stay there until you're completely sure the conversion is a success. You end up with two read-only filesystems sharing a partition, and you choose which one to go forward with. Cancelling at any point is trivial, even if the conversion process crashes.
Much of the operating system is proprietary, but I didn't think the kernel or HFS+ filesystem were included in that.
Also, btrfs is GPLv2. So that agrees with my point -- if they didn't mind using free software they could use that code.