For a moment, there, I thought you were going to say microphones. In a
universe of perfect fidelity, but where the living rooms are too small to
house a live orchestra, where does the aforementioned mechanical device,
the
acoustic properties of which are bound by the laws of physics, fit in?

To my mind, it 'fits' simply because it's the best that can be done (at
present) with regard
air borne sources. Clearly, not relevant to non air borne sources.
Regardless of source, as the final item in the chain is also mechanical, we
will always suffer the impact on the product that physics predicts.
(Until, of course, we are able (and willing) to plug our brains directly
into the system!)
The source signal, regardless of how it is captured, is the source signal
that we have to work with.
Undoubtedly it is tainted by many things (as well as the engineers
preference).
This thing of concern to me is how I can best reproduce the program content
of the master.
Whatever its 'quality', that is the reference by which we determine fidelity
within our systems is it not?
How well a system can faithfully reproduce the signal *presented to it* is
how I determine High Fidelity.

If microphones don't produce perfect fidelity, and if there is no
alternative means of converting sound into some sort of recording medium,
doesn't that mean that 'high fidelity' is actually concerned with
'listening
to microphones'?


No not at all. Hopefully clarified above.
In short: maintaining the fidelity of the master.

Paul