> I think the premise that professional salaries are determined entirely or even primarily by supply and demand is weak.

Your points about the perception of quality and digestible signals of quality are spot on, but what you're describing still boils down to supply and demand.

Supply and demand is a dynamic, complex system of the ever changing wants and needs of the producers (i.e. programmers producing code) and consumers (i.e. companies purchasing code) in a marketplace that determine prices (i.e. programmer salaries).

All considerations of programmer quality, signals of quality, type of programmer required for a specific job and so on are encapsulated within "supply and demand." It's an elegant system!