I generally feel like anything that requires your hands to be in the field of view (at anything near its current size) is an immediate non-starter, which is part of what ruined Air Tap for me. Plus it was just horribly imprecise.
We do stuff with our hands largely away from the center of our vision. You might make a brief glance to calibrate yourself, but e.g. grabbing a thing doesn't require constant focus on the thing through the entire act.
Air tap doesn't actually require your finger to be in field of view. Even in the Hololens 1, there is, in fact, a downward-facing camera for gestures whose field-of-camera extends all the way down to your waist.
We do stuff with our hands largely away from the center of our vision. You might make a brief glance to calibrate yourself, but e.g. grabbing a thing doesn't require constant focus on the thing through the entire act.