I think what you're missing is that this is likely in preparation for some sort of AR glasses and something smaller form factor than the Vision Pro that will require transparency so you can see through to the real world etc...
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If there is "some sort of AR glasses and something smaller form factor than the Vision Pro that will require transparency so you can see through to the real world", limit the transparent interface only to that device.
I look at my Mac screen to see what is on my Mac screen. Whether there is a tabletop, a window, or a cat behind my Mac screen is irrelevant. If I want to see what is behind it I would be looking it that instead of my screen
Repeat for application window, dialog box etc.
The only place translucence sort of kind of makes sense is for video content but even then, it still totally optional. My experience of watching a video is not really degraded if the play button is opaque.
As others have said, wanting to update the design simply because it looks cool is one thing. If that's the goal just say it. But Alan Dye's explanations do not inspire confidence. I get the impression that as in architecture, the chief audience of the design is not the user, but other designers.
Mark my words, this is coming. It’s doable (as an effect) - rear camera captures what’s behind the screen, face tracking adjusts perspective in real-time to make it seem transparent/have correct parallax.