why not just shoot bullseye with a head turn instead of turning your whole body as the modern locked-in stances require
Not that it would matter a whole lot in a very quickly developing situation like a gunfight, but the draw stroke to come from a standard 4:00 strong-side holster position to a bladed (er, reverse-bladed?) bullseye stance with the gun hand pointed directly to the side and rotated vertically is a lot more awkward than a forward presentation.
'Course, we don't practice that draw, either, so it would seem more awkward. The four-count isosceles draw stroke is pretty refined for fluidity and to allow you to fire the gun with effective hits at any point after it is rotated horizontal.
I'd think the equivalent "reverse-bladed" one-hand draw stroke would look something like this:
1) Hand on grip
2) Raise gun until it clears holster (most shooters would probably lean away from the gun and the target to aid clearance).
3) Lift elbow and extend arm out to side. When you reach full extension the gun will be horizontal and can be fired at the target, though "gangsta style" with the left side of the gun (if you're right handed) facing the ground. This would be a form of point shooting, sighting more or less over the knuckles.
4) Straighten your stance, rotate hand to right gun, obtain sight picture, and fire.
Until you've finished step four, you're really not in a bullseye stance.