Sorry, Prasanth, but that is actually not entirely true.
Learning proper singing technique actually can change and improve the sound and tone of a person's voice.Granted, the extent of the improvement will depend on the natural ability and range(wich can be maximized, but not increased, contrary to popular belief, when it comes to range and what your vocal folds can handle, ya get what you're born with ) of the singer, so you are partially correct.
In David A's case, it sounds like he has some training already and that is as good as it is going to get. I think that is part of why he is one of the least interesting of the top 7 to me. His voice is good, but not that good, and pleasant, but bland.I don't dislike him , I just find his vocals and performances to be uninteresting, safe, and predictable.
If Kristy for example were to improve her singing technique, she would have a better overall tone and control than she does currently. I can actually hear some of the mistakes as she makes them, and see/hear much of what she is doing that exaggerrates the nasal and brittle quality in her tone to the point of unpleasantness .It isn't her voice, it is her technique. She seems to be improving, so maybe someone is helping her with that. She could be reasonably good in time, and because she currently has to compensate for what she lacks vocally, she will be a better entertainer overall if she does learn to make the most of her ability.