I don't think BTRFS has all the features they want. One thing you can do with APFS is do an upgrade of an HFS+ partition to APFS. That might be difficult with BTRFS.
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.
Filesysytem snapshots are useful for recovery, but it's a very powerful mechanism for it. I.e. I think for a watch it's an overkill.
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.
BTRFS can upgrade from ext4 by slotting itself into free space and marking the old metadata as a special snapshot. The same thing should work on other filesystems with few new challenges.
Support for that has been dropped within Btrfs AFAIK. Also, "upgrading a filesystem in place" is more of a lottery than a feature that people should actually use. So many bad things can happen.
Well I did it a month ago, on a reasonably-updated system, at least.
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.
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.