In many places family practice docs serve as substitutes for general pediatricians. As the population gets older, family practice docs will be less able to see kids as old people demand their attention.(The updated workforce piece alludes to this in a rather vague way.)
In this context (and no offense intended to FPs) economists would refer to family practice docs as "substitute goods". When substitute goods become relatively scarce, the commodity consumers really want (i.e. pediatricians) becomes higher priced. 🙂