The goal is not to perfectly reproduce the sound of the piano, at least for me, because as you say where you are seated/positioned and the differing room acoustics/reflections of the venue will alter the sound for that particular vantage point compared to any other. There is no one "correct" sound; there are millions.
The goal in high fidelity, at least to me, is to instead attempt to faithfully reproduce the master tape. Fidelity, after all, means "faithfulness". [In a simplistic sense you can think of that 2ch master tape as "one specific vantage point the artists agreed had the sound/experience they wanted to convey".]
The musicians and their recording engineers worked as a team and painstakingly prepared that 2ch tape after listening to it dozens if not hundreds of versions using different mics, mic placement, different takes, EQ, reverb, effects, mixing, mastering, etc. and they apply their stamp of approval and signature to the final product: the 2ch master tape [these days often a hard drive actually].
They are the artists so
they get to decide how to make the "painting" and what hues to use,
not me, but in the end they finalized on one version and "signed their name to it".
So how do we test individual electrical components within our reproduction chain to see if their output is indeed faithful or not? One answer: Bypass tests. If you were testing if a piece of glass was truly transparent you look through it and then pull it away,
you bypass it, and see if you can detect a difference. The goal is for complete, 100% transparency: the incoming signal hitting the glass matches its output signal (according to human perception at least). Audio electronics and wires work similarly but with sound. If you can't detect when the device/wire in in or out of the signal path then you can't argue its output is compromised/poor or "low fidelity".
Transparency is not everyone's goal but it is mine and that of the brands I look to.