"you get a whole string of poor performers who seem to have no sense of their poorness"

A good point, Will. I'm pretty sure that there are many people whose musical- and self-awareness are so lacking that they don't actually realise they are bad performers. It's not their fault, it's not necessarily lack of practice, and it's not that they don't care about it, they just don't have the capacity to understand that they haven't got the necessary attributes to be a good performer.

I have a theory that, when they're performing, they aren't actually hearing themselves, in their head they're replaying the track they learned the song from, and imagining they sound the same.