Correct - Virtually all apps are "use only as intended" and are not scriptable - most apps cannot share data - WWW is heavily read/write asymmetric ( ie easy to read pages - impossible to change them yourself)
The entire model of software production has yet to reach its full potential. Personal computing still hasn't happened. It should be /very straightforward/ for a minimally-trained person to add features like this to their own music player.
I think this is completely accurate: the PC Revolution never finished, and it's on the backfoot now. The industry dropped live/moddable software for well-packaged applications.
A writable Web is a live and moddable Web. Bringing back the fight.
The "injected script" model of extensions is also to blame for poor scriptability on the Web. If extensions could export well-defined APIs which sites can query and consume, we'd have a much more powerful customization environment. As it is, extensions interact w/apps via the DOM
In every other situation we expect well-defined APIs with semvers when code interacts.
In Web Extensions, we yolo inject scripts into the page and monkey with the internal code. Complete lack of encapsulation or contract.