Apps do sleep tracking already, not everyone wants to wear a watch 24/7 either so would be useless to me and my wife. AOD is way better v a blank screen too.
I have zero desire to want to wear my Apple Watch to bed, and I never have. Sleep tracking isn’t of value to me, simply because my sleep is consistent and quality as is. But, it will be a huge feature when it does become native for the Apple Watch, in which Apple will definitely market it heavily. Also, given how ‘health oriented’ Apple is, I’m sure they’ve had sleep experts implementing studies for at least two years on the Apple Watch preparing for its debut.