Here's a thought - how many players today can play a melody, as opposed to playing a head?
Any piece, any piece, can be approached either way.
And think about it - are there any "standards", jazz originals, show tunes, whatever, that people don't know the melody to?
I think there's a general loss of interest in melody. The mathematics of harmony, characteristic rhythms and readily identifiable yet generalized tonal pallates per se have taken precedence, which is all well and good, but if all audiences want is a beat and/or energy and/or a certain "sound, and if all players are interested in is a good set of changes to blow on and/or a good groove to do it over and/or a certain "sound to do it with, is it any wonder that the same old same old keeps getting trotted out? After all, a ii-V is a ii-V, and if you're going to alter it every which way any way, what difference does it make what order they come in? And if everybody is looking for that good groove, what difference does it make waht you put on top of it? And if everybody's looking for certain sets of timbral qualities, what difference does it make what they're in the service of? Most players today don't sing (metaphorically), they play instruments. It's the difference between specific, personalized interaction and the operation of a machine for general public use.
One thing I've noticed - players who have an appreciation of pure melody tend to have a braoder repertoire than those who don't. And those who have a broad repertoire seem to have a better appreciation of melody than those who don't.
Not sure what, if anything, all this means, but hey.