What is it I'm failing to grasp?

Such a cool freakin' thread.  That's pretty much all I wanted to say, but I guess I have few tiny thoughts to add:

"Effortlessness."  That damn word.  I use it to mean the system is not pushing the sound to me, it's just letting it through.  I don't use that term to describe my listening experience, as in "listening for 72 straight hours was such an effortlessness experience."  So, the music itself could reflect lots of effort, but it's effortlessly communicated by your system.  Yeah, I'll take that.  But, honestly, maybe I'm using the term incorrectly.

To OP's point: I don't want my live music to be effortless.  I wanna admire the effort.  Not like watching Neil Young play an electric guitar; that's more than effort -- it's combat.  More like the focus, study, practice, and delivery that goes into a professional musical event, like Reckoning's Bird Song.

But the line of the thread is "It's something that is unamplified, has the highest quality acoustic instruments and most talented & trained singers - AKA the scenario where the matter of fidelity arises."  IOW, 75% of us are thinking of live music as the stuff we hear live -- via a PA system.  And, ya know, think about that.

I think it really depends on the type of music being listened to. It is a very difficult to reproduce a full symphony orchestra with fully frequency extension & full dynamic volume.  Conversely,  A truly good system can sound much better than many of the amplified rock concerts I’ve heard in the past 10+ years that are amplified & use the line array speaker design to supposedly offer the same balance & sound level throughout the venue. Most of them sound very hard, bright, dynamic but fatiguing. There are many good speakers out there that have very tone & body but few can actually present the dynamics of live music w/ full frequency range & realistic volume levels. For myself, good horn speakers with good tube amplification can do it. 

Different styles of music and recordings, some of which we don’t want to listen to for 24 hours. Put on AC/DC Live and you’ll enjoy the album and be satisfied when it’s finished. My system gave me all the energy and volume I could have wanted at the time but it won’t have been what it truly sounded like there. However you play some delicate vocal with a couple of accompanying instruments and you might find it sounds very lifelike and isn’t fatiguing. I guess systems can do both depending on the music genre and recording. For my main type of music tastes I can listen to my system for hours on end without feeling fatigued or having a headache, but I couldn’t put AC/DC on repeat at the kind of volumes that music requires to come alive. So for me I think a system can do both, it just depends what you’re listening to at the time. Having said all this, I don’t believe any system will truly be like you’re there, I just think it sounds as close to that to people who have the experience.

You are failing to grasp that the people who write this nonsense are idiots.

Neither to they have better hearing than us, nor even necessarily better systems.

They do have better imagination.

Recording in a studio is anything but natural from a permormer's point of view in an isolation booth with headphones on