Answer

The goal is to make an example of Facebook in order to put a brake on other applications deploying E2EE at scale; if Messenger has robust end-to-end encryption then the feature practically becomes an obligatory tickbox for all other and future messenger applications to adopt.

This will represent a loss of capability for state and police surveillance.

If they can stop the “mainstream” applications from becoming secure, then anyone who downloads an actually-secure messenger app to their phone will flag themselves as “interesting”.

For more discussion in this vein, see this previous blog posting: