I would modify your point #1 (decentralization must never be promoted to be a feature) to state: Openness, be it via a public API or freedom to take data out, should not be a central feature. Looking at the history of successful applications (Spotify, Facebook, Twitter) they have all offered a simple and well crafted experience, with open APIs at the start. I realize that they still have some form of APIs but they have been severely curtailed from their early days in all of my examples.

The openness of the APIs offers those interested the ability to create their own features on top of a base (Facebook games back in the day, Spotify's song previews on NYTimes "Best Music of 2020", etc) while still maintaining a central point that normal users can come to for the default experience. By letting power users build on top of it more users can be drawn in by the interested developers experimenting with new features and experiences.

In a way, these hip, modern, and flashy companies remind me of the ancient and dual unix philosophies of: "Do one thing, but do it well" and, for the power users, "give them just enough rope to hang themselves".