But new models to date have cost more than the previous ones to create, often by an order of magnitude, so the shoe metaphor falls apart.

A better metaphor would be oil and gas production, where existing oil and gas fields are either already finished (i.e. model is no longer SOTA -- no longer making a return on investment) or currently producing (SOTA inference -- making a return on investment). The key similarity with AI is new oil and gas fields are increasingly expensive to bring online because they are harder to make economical than the first ones we stumbled across bubbling up in the desert, and that's even with technological innovation. That is to say, the low hanging fruit is long gone.