Magic REQUIRES deception. Music does not. If the audience is not deceived there is no magic. There can be many things, but it fails to be MAGIC. Your "dead instrument" example is pointless.
Second, I do not see how your coin or billet example follows at all from the arguments presented. In fact, it is EXACTLY wrong. Even though a magician may use a large coin, it is still a coin and people know what they are, what they are used for, what their qualities are, and have experience with them that should make them seem "mundane" in characteristic even if unfamiliar with the specific coin. They are, after all, coins. (though Vernon did advocate using the types of coins people carry in their pockets, and anyone who has seen Allen Okawa work can testify to the difference in impact).
But what is a change bag?
When the bag was invented it was an item people saw when they attended church and, seeing it in a magic show, was probably kind of funny - as in ha ha funny. BUT people KNEW what it was, and that knowledge imbued a sense of the mundane onto the item. It was a common object. As spellbinder noted, it was a common object that did something secret, and the audience was MORE amazed at the impact of the effect. Smart performers used it to produce effects (in this case, mind reading) which were not identical to the method (switching).
But ask an audience today what a change bag is and what do you think the answer will be? Well, as the only place on the planet they will ever see one is at a magic store or in a magician's act (though I have actually seen one at one very old church's "museum") they will reply with "a magician prop" or (after it has been performed) "a magician's prop that switches one object for another."
Is this giving the best to our audience? Is the effort to just change the prop so over bearing that we would rather knowingly serve a audiences with an inferior technique?
People still write things down. So billets are fine. Having said that, the guy who figures out how to "read" information typed onto the iphone notepad will have a memorable tool in his arsenal, and that is NOT speculation. There is a magician in the Northeast who is performing an effect with a phone's contact list - I have people CONSTANTLY tell me about this performance. WHY? Because he is doing something amazing with something MUNDANE. Thankfully for the rest of us, writing on paper still happens. But should we stop, then yes, it will become as out of date as the bag.
You card analogy is, also, just silly. Again, cards (ESPECIALLY ones that look familiar) are mundane. People know what they are and what they do and because of that there can be amazement. Part of the reason card magic is so strong is because we do the extraordinary with the ordinary.
Is the change bag, today, ordinary?
And yes, people have picked cards before - magicians have found them. But this has nothing to do with the prop. But, to keep the discussion going, I should point out that if the audience knows how the picked card is found, there is no magic. ANd this may be where you wanted your argument to go: You can use a method "known" by your audience, as long as one's use still results in deception. THe key card remains a powerful method even though most laypeople know it, in essence. The problem with the change bag is that, in it's classic form, it tips it's method by its very existence. It would be like the card magician, before beginning his trick, reaching into his pocket and removing an odd backed card with the words "I will use this card to find your card" written on it, openly sticking it to the back of the selected card, and then looking for it.
Next, Unlike theater and disney, magic (as both Charles Reynolds and Teller have discussed) is not a willing suspension of disbelief. In the theater we see an orange disk and we choose to believe it is the sun. When we go to the magic show, do we choose to pretend we don't see the threads? OR - if we see the threads - "do we think, that's a pretty crappy magician, I can see the threads". In theater, one could very easily present the zombie ball without the cloth and it could be a profoundly magic-al experience. BUT without the deception, without an "agreeable illusion" it fails to be MAGIC. Need proof? Sit next to a 13 year old and ask what they think. I wonder if they are willing to "suspend their disbelief" when the rod peeks up past the cloth. If we saw the wires holding the witch in wicked, the impact of this special effect would be lessened, but we would not consider the production a failure. Can the same be said of a magic show?
And yes, magicians do enjoy watching magic shows. But is their EXPERIENCE one of MAGIC? Remember what that was like? The first time a sponge ball appeared in your hand. The first time you flipped over those two packets and you had separated them into all red and all black. The first time a well executed final load appeared under a cup you would swear your eyes never left.
That moment when your chest kind of hurts. Your head spins a little. When everything you know about the world seems both totally wrong and totally right all at the same instance.
Do we as magicians get that feeling when we buy a ticket to a magic show?
And yes, many who do buy tickets to see magic shows do know how some of the tricks work before the lights come on. But is this something we should be PROUD of? Isn't this an admission to failure out of the gate? Is this to be our giant justification? Is this to be used as a reason to play to the dumbest guy in the room, and not the smartest? Do you think copperfield became who he is by saying "this will fool some of the people, it's good enough, let's film it."
How hard is it to buy a different FREAKING PROP!!!!!!!
What's with you guys and your devotion to the change bag? It's like Stockholm syndrome, for Christ's sake.
Two more things:
1) You might want to check out pop culture references to magic. (I believe Wiseman even did a study on this.) How are magicians portrayed in the media. How is "being a magician" represented? Do you really believe that magic is an art held in esteem by the world at large? Do you believe that many even think of it as an art?
2) I think the most revealing line is your "marketing guy" reference. I don't know you, Bob, but I know I've seen a lot of "marketing guys" magic shows. I realize this is a broad brush, but MANY of them take an attitude, some explicitly stating, it's not about the tricks, it's about getting the show. Consequently, magic as an art may be of no issue to you. Doing the least required to get the money may be the only thing that matters to some with this attitude. "As long as the kids are having a good time, the parent's will continue to buy tickets."
But let's not deceive ourselves.
If what we are doing is "magic" it must be seen as impossible. For something to be seen as impossible, there must be no explanation.
Do you really think people who see the magician reach into the bag and take something out of it don't know EXACTLY what happened?
How is that "impossible"?
How is that "magic"?
Do you want to be a "magician" for some of the people in the room, or for all of them?