In the academic system, positions are globally accessible and therefore hyper-competitive. Moreover, headcount grows slowly as it depends on funding and attriti

In the academic system, positions are globally accessible and therefore hyper-competitive. Moreover, headcount grows slowly as it depends on funding and attrition through retirement/death. As such, demand outstrips supply. Strip everything extraneous away and that's the heart of it. If enough post-docs leave, it will reduce the demand for tenure-track positions. Eventually fewer PhDs will become post-docs, further decreasing demand.