There was a cheesy movie made back in the 80's (that I actually liked) called Crossroads that ended in a big "head cutting" guitar playing duel featuring Steve Vai. To me, it's every bit as good as the end to Rudy, Hoosiers, Invincible, or other sports movies.
Playing an instrument takes a different kind of athleticism, but you are an athlete none the less. You can have all the knowledge in the world, but can your fingers do what your mind tells them. I know the game of basketball better than Jeff knows music, but that doesn't mean I'm any good at it. I physically can't do the things a Kobe Bryant can do or a LeBron James. It's physically impossible for me.
What I've not seen mentioned in this thread much is talent and ability. In the academic world, we see this all the time. As a student, I could read something once and get an A or a B on the test where my wife would have to study for hours to get the same grade. I have a friend who has a photographic memory. He's amazing. He can read something once and recite it back. He's like a human encyclopedia.
Academic study of music is not for the slow minded. You have to be a pretty smart cookie to get it. It's not easy. Couple that with the fact that you have to have fine motor control and muscle memory that a surgeon would be jealous of, and you start to understand how difficult it is to be a "great" musician.
There is no doubt that anyone can pick up an instrument and learn to play some songs. I think my 5yo could do that. But there is a small group of players who have such an understanding of their instrument that they can do things most people can't, and could never do without dedicating countless hours trying to emulate one or two "licks" from a prolific player.
Here's the scene from the movie Crossroads:
Watch it, and see what you think. In my opinion, Steve Via wins hands down as he's nearly able to play a classical piece on the spot, without practice, that took Ralph Macchio studying at Julliard to learn (in the beginning of the movie, Ralph Macchio's character is studying at Julliard).
That's the scary thing. There are players out there with not a shred of musical academic knowledge who can play anything, on the spot. To me, those people are great. Anyone can practice something, but to play something right out of the air is what's amazing to me.