I have an early 20th c. recording of a Russian violinist playing Bach’s Sonatas and Partitias. The liner notes say he was a competition winning performer. By today’s standards he’s terrible. I only ever listened to it once.
My wife is a violinist. One day I loaded up the CD turntable with a variety of classical violinists. By chance, I loaded a CD of Itzhak Perlman, in his 20’s, just prior to a CD of Rachel Barton Pine, also in her 20’s. When the CD changed: “OMG,” my wife says, “Can you hear how much cleaner she plays?” It was pretty dramatic really.
There’s no way you can listen to those two recordings and not conclude that the standards of classical music have increased dramatically within our lifetime, and dramatically again before then and in recorded history (i.e., in recordings). There’s no reason to think folk musicians had standards any higher than classical musicians.
It is my belief (unsupported by any evidence) that this dramatic improvement is due primarily to recording technology. Today, I can record my playing, and use that to improve myself. I can record my teacher, and I can listen to recordings of pretty much any performer I want to. When my dad was growing up, the only people he could hear were others he knew, and only occasionally. Or maybe he had a few 78’s. 100 years ago, probably the only thing you could compare your own playing to was some other person, and only when you heard them, and only based on your memory of what you sounded like, having never heard for yourself what you really sound like.
There are stories of Bach seeking out the wolf in organs he tested, as a way of tweeking the organ tuners. So there’s no doubt that people have been able to hear “in tune” from “out”.
But there’s a difference between recording and live. And there’s a difference between sitting on a wolf and hearing a 1/16 note in context. I can’t tell you how many times I’ve made a recording of my wife at some paid gig, where I thought her playing was really good, where when we play the recording back at home she says, “That’s bad.” And she’s right. But it didn’t sound bad at all live and in person. So there’s some aspect of recording, per se, that allows us to evaluate more particularly how something sounds.