It's not so much of a problem of scale (it could be when we move beyond 16EiB of storage...): It's primarily about the features and functionality that ZFS offer

It's not so much of a problem of scale (it could be when we move beyond 16EiB of storage...): It's primarily about the features and functionality that ZFS offers, and the entire vertical integration of the storage stack helps that. From snapshots that make sense, to df working, to have no surprise-out-of-space error conditions, all the fundamental storage layers in ZFS working as expected, more redundancy types that btrfs can ever dream of (mirror, raidz[123], draid[123]), log/hot-spare devices, keeping all administrative details inside the pool where properties can inherit, actually having block devices without breaking CoW, per-dataset compression policies, quota/reservations, case-sensitivity, optional UTF-8 enforcement with optional normalization, It can go on and on. btrfs makes an attempt to compete with some of it, but after 14 years of development, it's still not even close to ZFS's first public release (which was 4~5 years after internal Sun development)