At some point, the turnover has to lead to "the blind leading the blind" with nobody having a clearer big picture view on the software they own. This can't be a productive way to run a company, but they seem to persist nonetheless. It may take many years, but I imagine their software will rot from within due to their hiring practices.

Exactly, Amazon practices the equivalent of a decimation of their workforce. This may even work in the initial years, but over time they'll quickly lose their best minds and the software will be unmaintainable.