"I've often wondered about applying the Mercury technique, including using
the real-deal Schoeps M201 microphones (they need restoration, but they're
still alive), with a modern high-resolution digital recording rig."

I'm ready when you are!

Would be a great experiment, but there are caveats. Recording systems are
systems; every part plays a role that needs to be complimentary to the
whole. Without the slight homogenization of tape, those treble peaks might
reveal themselves as rough terrain. Without the masking of noise, the
harmonic distortion may be grating. So much of recording, then as now, was
the overcoming of limitations, but different challenges for different eras.

Some of the challenge today is to find the magic when the tools are so
perfect. This does happen, without any particular emulation of golden-era
techniques. It's not all the technology, either. ("It's the guy, not the
gear.") I tell performers that when they sound good, I sound good. Of
course, the opposite is true, too, but that has to remain unsaid!

Speaking of orchestras, so much of the culture is different now from 50
years ago, it's not simple to point a finger at any one aspect. Generally
speaking, the players and directors are primarily concerned with clean
execution. This is understandable when most concerts are under-rehearsed and
the players often overworked with bazaar schedules - Bernstein one day,
Bruckner the next. Some of the finer points of sound and interpretation get
neglected, particularly to my perception dynamics, which is something that
recordings really need in order to overcome the limited sensory info.

Personally, I can enjoy less than perfect ensembles of the second or third
rank. In the US, these are budgetary differences as much as anything. The
majors are so routinized in their virtuosity, it is refreshing to hear
others rise to a challenge. That's when music gets made, and even limited
recording technique is adequate to convey that difference.