The correct explanation is the one you didn't cover here.

The correct explanation is the one you didn't cover here.

The absolute number of older developers keeps growing (because people stay in the industry more or less indefinitely) but the proportion shrinks because inexperienced devs get hired at much higher rates. Inexperienced devs make mistakes that create work for other inexperienced devs, creating the illusion of demand -- the more developers you hire, the more you seem to need.

Eventually this bubble will pop (if only because the absurdly inflated average salary for entry-level developers can't really be maintained, even with the market distortions that the VC model creates). You can't keep hiring folks for six figure salaries that actually produce negative value, even if nine out of ten companies go bankrupt in a year and the tenth one is some investor's charity case.