> Isn't this now the standard reality for most desktop and mobile OSs?
Not really, no. Because there isn't a comprehensive intent layer in the UI that is fully trusted and in which the user can grant capabilities.
Just a couple examples; if you download a PDF file with a mobile browser and want to open it in an app, the only option is to grant eternal (unless you manually revoke it) access to the local file system to the entire viewer app. A capability-aware UI would, at the "what do you want to do with this file?" system dialog grant a one time capability to read the particular file to the app the user picks. The capability persists as long as the app retains it. The app can't fork off a sub-process or send it to another app and clone the capability to the file; it would have to ask the user to do that. The mechanism for that trusted user request and response is where all UIs (and most CLIs) fail.
Designing a safe and useful capability desktop/mobile environment is hard. Modern UIs and CLIs are just papering over the details of actual objects in the privilege system. Icons and filenames are only a human-readable representation of objects accessible with capabilities and so there's a disconnect between UI representation, user intent, and user interactions. Solving that means replacing arbitrary custom UIs with trusted, object-aware system UIs that reveal the actual state of capabilities and objects instead of a simplified representation.
> Granting a permission based upon user approval is never secure anyway since if they want to see the porn / flashing lights they are going to click the button regardless.
The UI model would have to be significantly different with a lot of trusted components. Basically any UI element that can grant a capability to another app/process has to be part of the trusted system. That means a user-level app can't even ask for permissions to objects because it can't see them, know they exist, or refer to them in a way the system UI could grant a capability. Interaction between applications would be more like a smart clipboard; a user would open their camera app, take a picture, and grant a read capability to an app that's waiting for it.
There's a lot of 'convenience' lost in a pure capability system because there's no default trust in the good intentions of app developers.
There is a potential further development that allows convenience; formal proofs for application software that adheres to a particular data management policy. Say instagram wants to formally verify that photos taken automatically by the app (selfie button or whatever) can never be uploaded without user permission. Prove that the component for taking photos within the app drops all capabilities to the network and the main app itself before taking any photos; all it can do is fill up a particular folder/directory with new photos that are visible to the user, who can then trigger an upload (grant capability to a particular photo to the main app from a secure view of the directory) with another trusted interface. That trusted component could be granted a capability to the camera itself once the proof is verified.