You are missing the point. Music is not the kind of work that can be duplicated like a car and still provide the same effect. Music has no identically interchangeable parts. If I swap out the principal trumpet in my orchestra for another equally-qualified trumpet player, I'll get the same basic level of performance, but the subtleties of that performance will change. The way in which that player interprets their part will change. It may be something so simple as playing a particular passage a little louder or a little softer, or with a change in crescendo during a particular peak moment, but those individual subtleties change the qualities of the music, and thus, the emotional response that the music evokes in the listener. Music is not an objective art. It cannot be distilled down into a bunch of ones and zeroes and still retain all of the qualities that make it special. Even if you don't notice those things, there are people who do and those people look for those kinds of subtle qualities when they listen to a performance. It makes the experience more real for them.

The reality is that a recording is fixed and the performers on stage are not. The performers may (rightly-so) feel unnecessarily hamstrung if they have to try to fit their performances to a recorded track. Many very small production companies and some schools do it this way already (either out of lack of experience or out of financial need) but professional theatre groups have no business resorting to this tactic. It cheapens the experience and makes that $100 ticket you paid for worth that much less.
But what about pop music? Pop stars already use recordings, largely because there's so much movement on stage and parading around that your vocal ability is going to go into the toilet from all the movement - so lipsyncing to the recording is the only way it's going to sound even remotely passable. Choreographed performances in musicals are done differently - most songs are sung with a minimum of strenuous movement so that the vocals remain unaffected, and the more complex dance breaks are all instrumental - or if there IS a vocal part, it's either shouts or spoken words. So while I don't exactly condone it in pop music, with the attention on sex appeal, costuming, lighting, and special effects in a pop concert (rather than the music), recordings are necessary to retain what integrity is left of the musical performance.

Put simply, recordings have no place in live music or live theatre. Using a recording defeats the purpose of it being considered "live". I know I'd feel cheated if I paid for tickets to a musical and found that the musicians, singers, and actors on stage were all pre-recorded and that the people on stage just moved around without really doing anything.

The union may strike, which will only make things tougher on the musicians themselves in the end. What the union SHOULD do rather than just blindly strike (and I'm a card-carrying AFM member) is attempt to use their muscle to get licensing houses to enforce rules on "sticking to the score". It's already done heavily in terms of the text, why not the musicians? You can make cuts and edits for time or to adapt to the capability of the group, but if you're doing a full-on Broadway-level production with the appropriate talent, you shouldn't need to make cuts or edits.

A development like this is good for very small groups who have severely limited resources to hire musicians, and amateur-level productions should be allowed to do this. But at the level these people are performing at, coming by great musicians isn't hard, and the only reason the producers are doing it is pure greed. The production contracts should be what prohibits this kind of nonsense, because the audience CAN tell the difference between a live human and a synth, even if they are the most state-of-the-art. Even if you personally can't, I can, and I know lots of other non-musicians out there who can very clearly tell the difference between a synthesized performance and a live one.

The core argument here is not about money, although that's the facade it hides behind. While money is a component of the argument, the major sticking point is that this is handing off to computers a job that computers have no business doing. Computers have no means of expression, no emotion, and no understanding of the nuances of music to take black notes on a page and turn them into something greater than the sum of its parts. Sure, you can program algorithms and subroutines to take advantage of quantifiable, objective moments in music, but those occurrences in music are the exception, not the norm, and when things like this happen, the art is poorer for the experience.

Computers are excellent at jobs where the limits are finite, the ranges are quantifiable, and the duties are clear-cut. Music is none of those things. Humans, on the other hand, are excellent at those things. Leave to humans the jobs that humans can do. When computers fully develop sentience, critical thinking, decision-making ability, a sense of aesthetics, and a sense of right, wrong, and morality, THEN we'll talk about letting your PC sit in the principal violinist's chair.

Pretty simple. The AFM represents the same tired, defunct business model they always have (at least, when you look at the recording industry-related elements). I'm an AFM member, and pretty strongly disagree with the "party line" that they sometimes tow, and have pretty loudly spoken out sometimes against the mindless following that sometimes happens in our local when it comes to that. The real problem is that far too many working musicians can be bothered with things like, oh, actually bothering to read the details of what they're "supporting" because they're too busy trying to earn enough money to pay the rent. If you were to ask a lot of them individually, yeah, they'd certainly support a more freely-structured copyright system. AFM "calls to action" are worded so strongly that you'd be a fool -not- to want to go along with it. Trouble is that with the real legislation, nobody in their right mind (at least, who doesn't get a cut of the take) would support it if people actually bothered to read it. Don't get me wrong, the AFM does really good things in some ways - in my local's case, a central place to look for students to teach, easy job disbursement and a smartly-managed local in terms of personnel. In the national's case, making sure musicians get paid for the work they do, offering legal representation in the case of shoddy bookkeepers and crooked talent managers, and other bad people in the music business. The downside is that the national spams me every now and then with a "call to action" which usually isn't helpful. And their national agenda is just as crooked as the people they defend the musicians themselves against.