The Apple Watch can do authentication to some extent: Authy has an Apple Watch app, and there's Apple Pay. This is the kind of stuff I don't think Fitbit can easily duplicate, which is why I think they're going to struggle in the future.
The "to some extent" part is precisely a big part of my point when I mention fragmented standards and absence of critical mass. An incomplete killer application is not really "killer".
Other devices (e.g: Android wear with smart-lock and Google Pay) also suffer from the "to some extent" illness.
But you made a very good point about FitBit not being able to replicate. Their devices are too focused on health and notification. They don't even have the hardware to go beyond that.
I'd love it if I could securely store all of my passwords on an Apple Watch, which would require my fingerprint to unlock. But only if there was Bluetooth integration for macOS and USB devices for integration with random computers.
Another step would be a extra 1Password pin, of some other kind of something you know code. A 4 pad touch screen that could use a combination of short press, long press, force-touch press as a combination would be pretty cool, come to think of it.