No, Rob, I disagree. I really do think a person can teach himself to be a great artist, if he has the will. Look at Cezanne. Here's a guy who had no obvious early talent, no obvious midlife talent, and yet, by the end of his life he was being considered by many (including me) to be an absolutely pivotal artist in the whole history of art. Even Degas, who was an early bloomer and a genius with a pencil, considered Cezanne to be something far beyond the ordinary -- yet even at the end, after fifty years of hard work, Cezanne couldn't draw. What he did was stay with painting, day after day, to keep his brain alive, and eventually, found what he was doing.
I think what you consider to be talent is simply a wide band of learning effects which, in the case of the talented, often are gotten early in life. That is, a child, because he is told to do it, draws inside the lines, and somebody tells him or her, when she's three, gee, you're talented. That approval pushes the kid, who then gets better, and that results in a further push, and special attention, and the kid excels in drawing. It's a way of seeking status and approval. The interesting question is, can this be done when you're old? Some old artists (Rembrandt and Cezanne, among others) experienced a final blossoming in old age, in which their art moved to new levels. So, I think it's possible, but I'm not sure. I would agree that some basic physical requirements have to be met for top talents: for a photographer, you need good vision, intelligence, access to equipment, and so on. But I don't really think there there are great photographers wandering around, undiscovered, because they don't have access to the equipment. Access to the equipment is a prerequisite, in a way, to becoming talented. I've been told by a number of artists that if you can write with a pencil, you can learn to draw very, very well. Because of its use as a message carrier, drawing was once taught at West Point, and all the graduating students were expected to be at least capable (That's where Whistler got early instruction, before he was dismissed after failing chemistry, which generated a great later quote -- "If silicon were a gas, I'd be a general now.")
By the way, even you can learn to sing. I was once asked to leave the eighth grade choir because of my honking, but now I can sing certain kinds of songs reasonably well. What you need is music you like, sung in a range you can imitate, and then just try to match what the other voice is doing -- sing in harmony with it. Some people do this naturally. Other people have to do it as an intellectual thing, but it can be done.