My one philosophical takeaway from GTD is the idea of offloading the record keeping and logistics of tasks to some slower external system.

This allows the "RAM" or faster context of your thinking to be focused entirely on what you're doing.

Every system built on top of this (including GTD itself) is just an interpretation of this core idea that GTD points to that have unique selling points.

For example, ticketing systems inspired by toyota-scrum-agile externalize work in a way so that you can debug bottlenecks. GTD itself focuses on capture and filtering. Newer "second brain" stuff focuses on the knowledge management itself.

These external systems tend to grow until they require a dedicated cabal of priests to manage them. Since these systems measures all "progress", individuals then become incentivized to align with the priests (or become one).

Eventually the finger that points to the moon becomes the moon itself. The system was intended to allow you to focus on The Important Thing but instead becomes The Important Thing itself.