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OK... Just how do you know that the "inaudible" aliasing is not the actual perceived benefit of such filters? The alias content is time synchronous with the program content and is hard to pinpoint when played back by itself. That does not mean that it doesn't have an audible effect.
Go find an amplifier or preamp using loop feedback that has marginal phase margin in the amplifier response. Under a variety of conditions into real loads (cables that are not terminated in their characteristic impedance, which they almost never are, as one example), these amplifiers will often produce low level intermodulation distortion products high in the audio band. These are obviously time synchronous with the program content. What's that sound like? An awful lot like the descriptions I've read of how MQA sounds.
As for "Aliasing cannot be a problem if there are no signals to alias.", well, duh! This is why band limiting filters are used in the original A-D conversion. If there are no signals above half the sampling rate, no "anti-alias" filter is required. (Random noise will alias, but that's at the, ahem, noise floor.)
But, it does beg the question of just why you'd need various folding and encoding schemes to allow for higher sample rates if there is no information there.
What's the point of 96 KHz sample rates if there is no audio information above, for example, 20 KHz?
In the mixed signal world outside of audio, higher sample rates are usually employed to make the filtering easier, mostly because passive filters take up space, cost money, and modern engineers don't like to design them!
I'm probably wrong here, but I thought the same reasoning was true in audio. Well performing filters are difficult to implement with analog components, so you move the sampling rate higher to relax the requirements for these filters. There *may* be little or no program material above that magical 20 KHz, but at least the practical filter implementation is more easily attained. If there are useful signals above 20 KHz, they're captured as a kind of bonus.
Again, I'm probably just showing my ignorance, but I fail to see how this is some "post-Shannon" sampling theory. At best, it's clever management of what signals appear where and at what levels. Psychoacoustics, not sampling theory.
MQA may sound exquisite for all I know. The explanations do not.