Number of Apple Watch owners is much, much smaller than iPhone users. If you want that feature, I'd say "less than 12 inch"; so it works if you just pick up your phone with your Watch hand. 3 feet would allow someone to pick it up from my desk.
I'd like to pick it up with the other hand and still have the feature. Even today if you don't press the power button and leave the iPhone unlocked on your desk you're not safe.
Anyway that would only work with Apple Watch, we are a minority of users so they'd still need to provide another authentication method