Hi Julian,
A friend of mine used to go out & record local folk & jazz groups with a couple of decent microphones & a Revox cassette deck.
The quality of what could be put onto and replayed from 'metal' cassettes was quite startling. I agree that the process of putting pre-recorded music onto them introduces too many compromises but the format itself could be surprisingly good.
I understand your reluctance wrt FM and the big 'turn off' in a few years (scandalous imho) but would vehemently disagree with your view regarding its potential quality. Still, its your loss.