"Extensible and Sustainable" declares the rights, but not the obligations. Technical Collaboration Guidance/Principles doesn't say anything about that either. Forgive me if it is out of scope, but think is an important theme that is not (properly) mentioned.
Topic on Talk:Wikimedia Engineering Architecture Principles
These are not principles of architecture, so they are out of scope. The closest we have here is the FAST/CHANGE point. I agree we should have guidelines for planning processes and strategic decision making, but that's not what this document is about.
Let's please keep the discussion on phabricator, though: phab:T220657. If you want to discuss this further, please copy our conversation there.