Greg- good point about the status of professional musicians- it reflects cricket's gentlemen and players exactly. (For non- cricketers, before about 1960, amateur cricketers- the gentlemen- were considered of higher status than the paid professionals.) But you can bet that this was a class distinction, rather than anything to do with money. Many nominal "gentlemen" earned much more from cricket- though not directly paid on a match basis- than did the "players". Similarly, unpaid musicians would be divided into those of too high status to be insulted by a direct fee, and those whose services were not required.

As for Jim's excellent discussion of the provenance of known recent songs and tunes, also remember that some at least of these could have been regarded initially as reworkings of existing material, so not pure compositions as such. And particularly with tunes, they could become attached to a personality by association, rather than being their compositions.

As for copyright, the person who invented the integrated circuit, which has done more good (and harm) than any song or other work of art, got 25 years protection from the date of registration. I can't honestly see why pop songs should be expected to bankroll the grandchildren of their composers.