Why should we think of "what microphones heard " as a standard

And that's not even pro tape decks and no high end microphones, I suppose. It says a lot about how bad most commercial recordings are.

I think it's generally accepted that both microphones and loudspeakers (mechanical transducers) are by far the weakest links in the audio chain. 

Both can feature differing technologies and both are still far from a desirable  <1% distortion. But we can measure this distortion and we can hopefully keep  reducing it further.

Studios spend thousands of dollar on good microphones to capture the real sound and hundreds of thousands on gear to alter the sound to the producer's taste.  Ask anyone who actually worked in recording studios.  There are exceptions of course - MA recordings, Water Lily, Reference Recordings.  But those are really exceptions. 

Microphones certainly are transducers, the recording equivalent of the phono cartridge in playback. They therefore are more inherently prone to distortion than are electronics, but the way in which outboard processing is used by the vast majority of recording engineers (aside from guys like Kav Alexander of Water Lily), the electronics are actually more responsible for the lack of audiophile-quality sound found on most of our LP's and CD's than are the microphones used. Ask Ralph Karsen (atmasphere) if you don't believe me ;-) .

Yeah, microphones and speakers - the beginning and the end. As important as everything else in between is, I would get it right first.