Fairphone modules are regular phone components (like the camera, antenna… etc) that have been cleverly repackaged by Fairphone to replace the use of the original connectors by pogo pins and to make their removal/replacement easier. You can see an example of someone disassembling his camera module here.
wtf
Reducing e-waste has been one of Fairphone’s goals for the FP2 from the ground up. The point Paul was making was from the point of view of someone that does not need to upgrade his phone’s specs every other week (which will become more common now people realise our phones’ hardware specs are highly overkill given how we use them). Reducing e-waste for FP2 is done by offering the possibility to easily and painlessly replacing a broken component (like a malfunctioning core module, a worn-out battery, a broken camera…) with spare parts to prevent people from having to buy a whole new phone when a single component breaks.
As I have mentioned in my previous comment, the Android OS has not been thought with modularity in mind. Upgrading only the core module would mean a new version of Android, complete with its tests, bugs and update system would have to be made for your new phone that consists of a high-tech proc and outdated components (that might not even be properly supported by the proc).
That would only increase the number of “phones” FP would have to support. The costs both for you and Fairphone would rise accordingly. This also applies to a later part of your comment :
I’m not sure how this relates to the matter at hand.
Major Android versions are functional updates, while anterior versions still receive security patches. It’s way better to have a regularly patched 5.1 than a 7.0 that won’t be fixed for months (especially with the last privilege escalation exploit that has been discovered recently).
What’s more, latest Android updates have been overwhelmingly Google Services updates, the Android basis itself does not receive much love (except maybe for the privacy center and doze mode in Marshmallow, which existed on Cyanogenmod years prior to 6.0, and multi-window mode in Nougat that existed on other Cyanogen-based roms (like Blisspop)).
Upgrading to more recent Android versions is not only a matter of manpower, time and money. There are functional requirements that need to be checked, and agreements that need to be made with third parties that hinder the process (for example, if you want to know why the FP2 will likely never see Nougat you can read this. The FP2 is a msm8974 device).