Even that 50% number is questionable. Think about it- if you have or had fairly dense hair, and you plucked out every other hair, would it even be noticeable? I think not. It's all about uniformity. If you lose a bunch of hairs in one area, say 2/3 or more hairs at the temple or at the crown, then it becomes noticeable, because then you can see the scalp a little.
Either way, it's a good rule of thumb. I would say I lost at least 75%, that's 3/4, hairs in the front middle of my scalp, and nobody knows what I'm talking about when I tell them I'm losing my hair. But if I part my hair in the middle with my hands, then it is evident.
A good hair transplant would remove thick, healthy hairs from a non-dht susceptible area of the head- most likely from the back- and place them on the balding areas of the scalp, spread out enough so that it looks "real" or "natural".